1. Preface
2. Introduction to Human Biology and the Scientific Method
1. Introduction
2. Structural Organization of the Human Body
3. Functions of Human Life
4. Classification of Organisms
5. The Process of Science
3. Chemistry and Life
1. Introduction
2. The Building Blocks of Molecules
3. The Chemical and Physical Properties of Water
4. Biological Macromolecules
4. Cells
1. Introduction
2. Prokaryotic and Eukaryotic Cells
3. A More Detailed Look at Eukaryotic Cells
4. A More Detailed Look At The Cell Membrane
5. Passive Transport Mechanisms
6. Active Transport Mechanisms
5. DNA and Gene Expression
1. Introduction to the Central Dogma of Molecular Biology
2. DNA and RNA
3. The Basics of DNA Replication
4. ‘Transcription
5. Translation
6. Digestive System
1. Homeostasis
2. The Digestive System
7. Energy Considerations
1. Introduction to Metabolism
2. Energy and Metabolism
3. Glycolysis
4. The Transition Reaction, Citric Acid/Kreb's Cycle and
Electron Transport Chain/Oxidative Phosphorylation
o. Fermentation
8. Blood
1. Introduction to the Cardiovascular System - Blood
2. An Overview of Blood
3. Erythrocytes
4. Blood Typing and Transfusions
9. Heart
1. Introduction to the Cardiovascular System - Heart
2. Heart Anatomy
3. Cardiac Muscle and Electrical Activity
4. Cardiac Cycle
10. Blood Vessels
1. Introduction to the Cardiovascular System - Blood Vessels
and Circulation
2. Structure and Function of Blood Vessels
11. Respiratory System
1. Introduction to the Respiratory System
3. Gas Pressure, Volume, and Breathing
4. Gas Exchange
5. Transport of Gases
12. Hormones
1. Endocrine System
13. Urinary System
1. Introduction to the Urinary System
2. Urinary System Anatomy and Function
3. Hormonal Control of Urine Concentration
14. Mitosis and Meiosis
1. Introduction to Cell Division
2. Chromosomes and the Genome
3. The Cell Cycle
4. Meiosis and Genetic Variation
15. Reproductive Systems
1. Introduction to the Reproductive Systems
3. Female Reproductive Anatomy and Physiology; Gestation
and Labor
16. Skeletal System
1. Introduction to Bone Tissue
2. Functions of the Skeletal System
3. Bone Structure
4. Bone Formation and Development
17. Muscles and Movement
1. Muscle Contraction and Locomotion
18. Nervous System
1. Introduction to the Nervous System
2. Neurons and Glial Cells
3. How Neurons Communicate
4. The Central and Peripheral Nervous Systems
19. Special Senses
1. Introduction to the Special Senses
2. Taste and Smell
3. Hearing and Vestibular Sensation
4. Vision
20. Immune System
1. Introduction to the Immune System
2. Innate Immunity
3. Adaptive Immunity
Preface
This book is derived from three OpenStax resources: Biology, Concepts of
Biology, and Anatomy and Physiology. It has been extensively edited so
that the chapter order and content is appropriate for a non-majors human
biology course.
Welcome to Human Biology, a textbook created utilizing OpenStax
resources. This textbook has been created with several goals in mind:
accessibility, customization, and student engagement—all while
encouraging students toward high levels of academic scholarship. Students
will find that this textbook offers a strong introduction to human biology in
an accessible format.
About OpenStax College
OpenStax College is a non-profit organization committed to improving
student access to quality learning materials. Their free textbooks are
developed and peer-reviewed by educators to ensure they are readable,
accurate, and meet the scope and sequence requirements of today’s college
courses. Unlike traditional textbooks, OpenStax College resources live
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partnerships with companies and foundations committed to reducing costs
for students, OpenStax College is working to improve access to higher
education for all. OpenStax College is an initiative of Rice University and is
made possible through the generous support of several philanthropic
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About OpenStax College's Resources
OpenStax College resources provide quality academic instruction. Three
key features set our materials apart from others: they can be customized by
instructors for each class, they are a “living” resource that grows online
through contributions from science educators, and they are available free or
for minimal cost. The materials for this book were compiled and
customized by Willy Cushwa, with valuable editorial assistance provided
by Jamey Marsh. Please send any content suggestions and/or corrections to
Willy Cushwa at wcushwa@clark.edu.
To broaden access and encourage community curation, our text books are
“open source” licensed under a Creative Commons Attribution (CC-BY)
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keep it current and relevant for today’s students. Submit your suggestions to
info@openstaxcollege.org, and check in on edition status, alternate
versions, errata, and news on the StaxDash at http://openstaxcollege.org.
Cost
Our textbooks are available for free online, and in low-cost print and e-book
editions.
About Our Team
Concepts of Biology would not be possible if not for the tremendous
contributions of the authors and community reviewing team
Senior Contributors
Samantha Fowler Clayton State University
Rebecca Roush Sandhills Community College
James Wise Hampton University
Faculty Contributors and Reviewers
Mark Belk
Lisa Boggs
Sherryl Broverman
David Byres
Aaron Cassill
Karen Champ
Sue Chaplin
Diane Day
Jean DeSaix
David Hunnicutt
Barbara Kuehner
Brenda Leady
Bernie Marcus
Flora Mhlanga
Madeline Mignone
Elizabeth Nash
Mark Newton
Diana Oliveras
Ann Paterson
Brigham Young University
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College of Central Florida
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Introduction
class="introduction"
Blood Pressure
A basic
understandin
g of medical
procedures
allows you to
better
understand
information
collected by
medical
professionals.
(credit: Bryan
Mason/flickr)
Note:
Chapter Objectives
After studying this chapter, you will be able to:
e Describe the structure of the body, from simplest to most complex, in
terms of the six levels of organization
e List characteristics of human life
e Define homeostasis and explain its importance to normal human
functioning
Though you may approach a course in human biology strictly as a
requirement for obtaining your degree, the knowledge you gain in this
course will serve you well in many aspects of your life. An understanding
of your body and how it works can benefit your own health. Familiarity
with the human body can help you make healthful choices and prompt you
to take appropriate action when signs of illness arise. Your knowledge in
this field will help you understand news about nutrition, medications,
medical devices, and procedures. This knowledge will also help you
understand genetic and infectious diseases. At some point, everyone will
have a problem with some aspect of his or her body and your knowledge
can help you to be a better parent, spouse, partner, friend, or caregiver.
This chapter begins with an overview of anatomy and physiology. It then
covers the characteristics of life and how the body works to maintain stable
conditions.
Structural Organization of the Human Body
By the end of this section, you will be able to:
¢ Describe the structure of the human body in terms of six levels of
organization
e List the eleven organ systems of the human body and identify at least
one organ and one major function of each
Before you begin to study the different structures and functions of the
human body, it is helpful to consider its basic architecture; that is, how its
smallest parts are assembled into larger structures. It is convenient to
consider the structures of the body in terms of fundamental levels of
organization that increase in complexity: subatomic particles (e.g. protons,
neutrons, and electrons), atoms, molecules, macromolecules (e.g.
carbohydrates, lipids, proteins, and nucleic acids) organelles, cells, tissues,
organs, organ systems, and organisms ((link]).
Levels of Structural Organization of the Human Body
Oxygen atom
Chemical level
Atoms bond to form
molecules with three-
dimensional structures.
Jd Water molecule
Smooth muscle cell
Cellular level
A variety of molecules
combine to form the
fluid and organelles
of a body cell.
Organelle
Cell fluid
Organelle
Smooth muscle tissue
Tissue level
A community of similar
cells form a body tissue.
Bladder
Organ level
Two or more different tissues
combine to form an organ.
Skeletal
muscle
Urinary tract system
Kidney
Organ system level
Ureter Two or more organs work
closely together to perform
he functi \f .
Bladder the functions of a body system.
A, Ureth ra
1"
Organismal level
Many organ system work harmoniously
together to perform the functions of an J
independent organism. fo
The organization of the body often is discussed in terms of six
distinct levels of increasing complexity, from the smallest
chemical building blocks to a unique human organism. Note:
The macromolecule level (which is located between molecules
and organelles) isn't shown.
The Levels of Organization
To study the chemical level of organization, scientists consider the simplest
building blocks of matter: subatomic particles, atoms and molecules. All
matter in the universe is composed of one or more unique pure substances
called elements, familiar examples of which are hydrogen, oxygen, carbon,
nitrogen, calcium, and iron. The smallest unit of any of these pure
substances (elements) is an atom. Atoms are made up of subatomic particles
such as the proton, electron and neutron. Two or more atoms combine to
form a molecule, such as the water molecules, proteins, and sugars found in
living things. Molecules are the chemical building blocks of all body
structures.
A cell is the smallest independently functioning unit of a living organism.
Even bacteria, which are extremely small, independently-living organisms,
have a cellular structure. Each bacterium is a single cell. All living
structures of human anatomy contain cells, and almost all functions of
human physiology are performed in cells or are initiated by cells.
A human cell typically consists of flexible membranes that enclose
cytoplasm, a water-based cellular fluid together with a variety of tiny
functioning units called organelles. In humans, as in all organisms, cells
perform all functions of life. A tissue is a group of many similar cells
(though sometimes composed of a few related types) that work together to
perform a specific function. An organ is an anatomically distinct structure
of the body composed of two or more tissue types. Each organ performs one
or more specific physiological functions. An organ system is a group of
organs that work together to perform major functions or meet physiological
needs of the body.
Figures 2 and 3 below show the eleven distinct organ systems in the human
body. Assigning organs to organ systems can be imprecise since organs that
“belong” to one system can also have functions integral to another system.
In fact, most organs contribute to more than one system. In this course, we
will discuss some, but not all, of these organ systems.
Organ Systems of the Human Body
Integumentary System Skeletal System
¢ Encloses internal * Supports the body
body structures ¢ Enables movement
* Site of many (with muscular
sensory receptors system)
Muscular System Nervous System
¢ Enables movement * Detects and
(with skeletal system) processes sensory
¢ Helps maintain body information
temperature * Activates bodily
responses
Skeletal
muscles
Endocrine System Cardiovascular System
Pituitary
gland * Secretes hormones * Delivers oxygen
. ¢ Regulates bodily and nutrients to
Thyroid processes tissues
gland * Equalizes
temperature in
the body
Pancreas
Adrenal
glands
Blood
vessels
Ovaries
Organs that work together are grouped into organ
systems.
Organ Systems of the Human Body (continued)
Lymphatic System
¢ Returns fluid to
blood
¢ Defends against
pathogens
Digestive System
¢ Processes food for
use by the body
« Removes wastes
from undigested
food
Stomach
Liver
Gall
bladder
Large
intestine
Small
intestine
Male Reproductive
System
¢ Produces sex
hormones and
gametes
* Delivers gametes
to female
Epididymis
Respiratory System
* Removes carbon
dioxide from the
body
* Delivers oxygen
to blood
Nasal passage
Trachea
Lungs
Urinary System
* Controls water
balance in the
body
« Removes wastes
from blood and
Kidneys excretes them
Urinary
bladder
Female Reproductive
System
* Produces sex
Mammary hormones
glands and gametes
* Supports embryo/
fetus until birth
* Produces milk for
: infant
Ovaries
Organs that work together are grouped into organ
systems.
The organism level is the highest level of organization. An organism is a
living being that has a cellular structure and that can independently perform
all physiologic functions necessary for life. In multicellular organisms,
including humans, all cells, tissues, organs, and organ systems of the body
work together to maintain the life and health of the organism.
Chapter Review
Life processes of the human body are maintained at several levels of
structural organization. These include the chemical, cellular, tissue, organ,
organ system, and the organism level. Higher levels of organization are
built from lower levels. Therefore, subatomic particles combine to produce
atoms, atoms combine to produce molecules, molecules combine to produce
macromolecules, macromolecules contribute to the formation of organelles
which combine to form cells, cells combine to form tissues, tissues combine
to form organs, organs combine to form organ systems, and organ systems
combine to form organisms.
Review Questions
Exercise:
Problem:
The smallest independently functioning unit of an organism is a(n)
a. cell
b. molecule
c. organ
d. tissue
Solution:
A
Exercise:
Problem:
A collection of similar tissues that performs a specific function is an
a. organ
b. organelle
c. organism
d. organ system
Solution:
A
Exercise:
Problem:
The body system responsible for processing food, absorbing nutrients,
and expelling wastes is the
a. cardiovascular system
b. endocrine system
c. muscular system
d. digestive system
Solution:
D
CRITICAL THINKING QUESTIONS
Exercise:
Problem:Name the six levels of organization of the human body.
Solution:
Chemical, cellular, tissue, organ, organ system, organism.
Exercise:
Problem:
The female ovaries and the male testes are a part of which body
system? Can these organs be members of more than one organ system?
Why or why not?
Solution:
The female ovaries and the male testes are parts of the reproductive
system. But they also secrete hormones, as does the endocrine system,
therefore ovaries and testes function within both the endocrine and
reproductive systems.
Glossary
cell
smallest independently functioning unit of all organisms; in animals, a
cell contains cytoplasm, composed of fluid and organelles
organ
functionally distinct structure composed of two or more types of
tissues
organ system
group of organs that work together to carry out a particular function
organism
living being that has a cellular structure and that can independently
perform all physiologic functions necessary for life
tissue
group of similar or closely related cells that act together to perform a
specific function
Functions of Human Life
By the end of this section, you will be able to:
e Explain the importance of organization to the function of the human
organism
e Distinguish between metabolism, anabolism, and catabolism
e Provide at least two examples of human responsiveness and human
movement
e Compare and contrast growth, differentiation, and reproduction
The different organ systems each have different functions and therefore
unique roles to perform in physiology. These many functions can be
summarized in terms of a few that we might consider definitive of human
life: organization, metabolism, responsiveness, homeostasis, adaptation,
movement, development, and reproduction.
Organization
A human body consists of trillions of cells organized in a way that
maintains distinct internal compartments. These compartments keep body
cells separated from external environmental threats and keep the cells moist
and nourished. They also separate internal body fluids from the countless
microorganisms that grow on body surfaces, including the lining of certain
tracts, or passageways. The intestinal tract, for example, is home to even
more bacteria cells than the total of all human cells in the body, yet these
bacteria are outside the body and cannot be allowed to circulate freely
inside the body.
Cells, for example, have a cell membrane (also referred to as the plasma
membrane) that keeps the intracellular environment—the fluids and
organelles—separate from the extracellular environment. Blood vessels
keep blood inside a closed circulatory system, and nerves and muscles are
wrapped in connective tissue sheaths that separate them from surrounding
structures. In the chest and abdomen, a variety of internal membranes keep
major organs such as the lungs, heart, and kidneys separate from others.
The body’s largest organ system is the integumentary system, which
includes the skin and its associated structures, such as hair and nails. The
surface tissue of skin is a barrier that protects internal structures and fluids
from potentially harmful microorganisms and other toxins.
Metabolism
The first law of thermodynamics holds that energy can neither be created
nor destroyed—it can only change form. Your basic function as an
organism is to consume (ingest) molecules in the foods you eat, convert
some of it into fuel for movement, sustain your body functions, and build
and maintain your body structures. There are two types of reactions that
accomplish this: anabolism and catabolism.
e Anabolism is the process whereby smaller, simpler molecules are
combined into larger, more complex substances. Your body can
assemble, by utilizing energy, the complex chemicals it needs by
combining small molecules derived from the foods you eat
¢ Catabolism is the process by which larger more complex substances
are broken down into smaller simpler molecules. Catabolism releases
energy. The complex molecules found in foods are broken down so the
body can use their parts to assemble the structures and substances
needed for life.
Taken together, these two processes are called metabolism. Metabolism is
the sum of all anabolic and catabolic reactions that take place in the body
({link]). Both anabolism and catabolism occur simultaneously and
continuously to keep you alive.
Metabolism
Catabolism | 9 Anabolism
Releases O Requires
energy im o energy @
Food
Anabolic reactions are building
reactions, and they consume
energy. Catabolic reactions
break materials down and
release energy. Metabolism
includes both anabolic and
catabolic reactions.
Every cell in your body makes use of a chemical compound, adenosine
triphosphate (ATP), to store and release energy. Think of ATP as the
energy "currency" of the cell. If energy is needed for something to happen
in the cell, then ATP is used to "pay the energy bill". The cell stores energy
in the synthesis (anabolism) of ATP, then moves the ATP molecules to the
location where energy is needed to fuel cellular activities. Then the ATP is
broken down (catabolism) and a controlled amount of energy is released,
which is used by the cell to perform a particular job.
Responsiveness
Responsiveness is the ability of an organism to adjust to changes in its
internal and external environments. An example of responsiveness to
external stimuli could include moving toward sources of food and water and
away from perceived dangers. Changes in an organism’s internal
environment, such as increased body temperature, can cause the responses
of sweating and the dilation of blood vessels in the skin in order to decrease
body temperature, as shown by the runners in [link].
Homeostasis
To function properly, cells require appropriate conditions such as proper
temperature, pH, and concentrations of diverse chemicals. These conditions
may, however, change from one moment to the next. Organisms are able to
maintain internal conditions within a narrow range almost constantly,
despite environmental changes, through a process called homeostasis or
“steady state”—the ability of an organism to maintain constant internal
conditions. For example, many organisms regulate their body temperature
in a process known as thermoregulation. Organisms that live in cold
climates, such as the polar bear, have body structures that help them
withstand low temperatures and conserve body heat. In hot climates,
organisms have methods (such as perspiration in humans or panting in
dogs) that help them to shed excess body heat. As we discuss organ
systems, the concept of homeostasis will be critically important to
remember.
Adaptation
All living organisms exhibit a “fit” to their environment. Biologists refer to
this fit as adaptation and it is a consequence of evolution by natural
selection (i.e. survival of the fittest), which operates in every lineage of
reproducing organisms. Examples of adaptations are diverse and unique,
from heat-resistant bacteria that live in boiling hot springs to the tongue
length of a nectar-feeding moth that matches the size of the flower from
which it feeds. All adaptations enhance the reproductive potential of the
individual exhibiting them, including their ability to survive to reproduce.
Adaptations are not constant. As an environment changes, natural selection
causes the characteristics of the individuals in a population to track those
changes.
Movement
Human movement includes not only actions at the joints of the body, but
also the motion of individual organs and even individual cells. As you read
these words, red and white blood cells are moving throughout your body,
muscle cells are contracting and relaxing to maintain your posture and to
focus your vision, and glands are secreting chemicals to regulate body
functions. Your body is coordinating the action of entire muscle groups to
enable you to move air into and out of your lungs, to push blood throughout
your body, and to propel the food you have eaten through your digestive
tract. Consciously, of course, you contract your skeletal muscles to move
the bones of your skeleton to get from one place to another (as the runners
are doing in [link]), and to carry out all of the activities of your daily life.
Marathon Runners
Runners demonstrate two characteristics of living
humans—responsiveness and movement. Anatomic
structures and physiological processes allow runners
to coordinate the action of muscle groups and sweat in
response to rising internal body temperature. (credit:
Phil Roeder/flickr)
Development, growth and reproduction
Development is all of the changes the body goes through in life.
Development includes the processes of differentiation, growth, and renewal.
Growth is the increase in body size. Humans, like all multicellular
organisms, grow by increasing the number of existing cells, increasing the
amount of non-cellular material around cells (such as mineral deposits in
bone), and, within very narrow limits, increasing the size of existing cells.
Reproduction is the formation of a new organism from parent organisms.
In humans, reproduction is carried out by the male and female reproductive
systems. Because death will come to all complex organisms, without
reproduction, the line of organisms would end.
Chapter Review
Most processes that occur in the human body are not consciously
controlled. They occur continuously to build, maintain, and sustain life.
These processes include: organization, in terms of the maintenance of
essential body boundaries; metabolism, including energy transfer via
anabolic and catabolic reactions; responsiveness; homeostasis; adaptation;
movement; and growth, differentiation, reproduction, and renewal.
Interactive Link Questions
Exercise:
Problem:
View this animation to learn more about metabolic processes. What
kind of catabolism occurs in the heart?
Solution:
Fatty acid catabolism.
Review Questions
Exercise:
Problem: Metabolism can be defined as the
a. adjustment by an organism to external or internal changes
b. process whereby all unspecialized cells become specialized to
perform distinct functions
c. process whereby new cells are formed to replace worn-out cells
d. sum of all chemical reactions in an organism
Solution:
D
Exercise:
Problem:
Adenosine triphosphate (ATP) is an important molecule because it
a. is the result of catabolism
b. releases energy in uncontrolled bursts
c. provides energy for use by body cells
d. All of the above
Solution:
C
Exercise:
Problem:
Cancer cells can be characterized as “generic” cells that perform no
specialized body function. Thus cancer cells lack
a. differentiation
b. reproduction
c. responsiveness
d. both reproduction and responsiveness
Solution:
A
CRITICAL THINKING QUESTIONS
Exercise:
Problem:
Explain why the smell of smoke when you are sitting at a campfire
does not trigger alarm, but the smell of smoke in your residence hall
does.
Solution:
When you are sitting at a campfire, your sense of smell adapts to the
smell of smoke. Only if that smell were to suddenly and dramatically
intensify would you be likely to notice and respond. In contrast, the
smell of even a trace of smoke would be new and highly unusual in
your residence hall, and would be perceived as danger.
Exercise:
Problem:
Identify three different ways that growth can occur in the human body.
Solution:
Growth can occur by increasing the number of existing cells,
increasing the size of existing cells, or increasing the amount of non-
cellular material around cells.
Glossary
anabolism
assembly of more complex molecules from simpler molecules
catabolism
breaking down of more complex molecules into simpler molecules
development
changes an organism goes through during its life
differentiation
process by which unspecialized cells become specialized in structure
and function
growth
process of increasing in size
metabolism
sum of all of the body’s chemical reactions
renewal
process by which worn-out cells are replaced
reproduction
process by which new organisms are generated
responsiveness
ability of an organisms or a system to adjust to changes in conditions
Classification of Organisms
By the end of this section, you will be able to:
¢ Describe the levels of organization among living things
e State the domain, kingdom, genus, and species for humans
The Diversity of Life
In the 18th century, a scientist named Carl Linnaeus first proposed
organisms are collected together into groups at the highest level. The
current taxonomic system now has eight levels in its hierarchy, from lowest
to highest: species, genus, family, order, class, phylum, kingdom, domain.
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Eukarya Dog Wolf Coyote Fox S Sn h
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KINGDOM
Lion Mouse Whale Fish Earthworm
Animalia Dog Wet Coes Fa Sn Moth
Seal Human Bat ake
PHYLUM
Fox Lion Mouse Whale Fish
Chordata
Dog. “Wolf, “Coyote Seal Human Bat Snake
CLASS Lion Mouse Whale
Mammalia Dog a Fox Seal Human Bat
ORDER
Carnivora
Lion
Dog Wolf Coyote Fox Seal
FAMILY
Canidae Dog Wolf Coyote Fox
GENUS
Canis Dog Wolf Coyote
SPECIES
Canis lupus Dog a
This diagram shows the levels of taxonomic hierarchy for a
dog, from the broadest category—domain—to the most
specific—species. Notice that humans and dogs diverge at
the level of order. Humans are classified in the following
levels: order-Primates; family-Hominidae; genus: Homo;
species- Homo sapiens; scientific/binomial name: Homo
Sapiens.
In addition to the hierarchical taxonomic system, Linnaeus was the first to
name organisms using two unique names, now called the binomial naming
system. Before Linnaeus, the use of common names to refer to organisms
caused confusion because there were regional differences in these common
names. Binomial names (also called scientific names) consist of the genus
name (which is capitalized) and the species name (all lower-case). Both
names are set in italics when they are printed. Every species is given a
unique binomial which is recognized the world over, so that a scientist in
any location can know which organism is being referred to. For example,
the North American blue jay is known uniquely as Cyanocitta cristata. Our
Own species is Homo sapiens.
Note:
Glossary
atom
a basic unit of matter that cannot be broken down by normal chemical
reactions
biology
the study of living organisms and their interactions with one another
and their environments
biosphere
a collection of all ecosystems on Earth
cell
the smallest fundamental unit of structure and function in living things
community
a set of populations inhabiting a particular area
ecosystem
all living things in a particular area together with the abiotic, nonliving
parts of that environment
eukaryote
an organism with cells that have nuclei and membrane-bound
organelles
evolution
the process of gradual change in a population that can also lead to new
species arising from older species
homeostasis
the ability of an organism to maintain constant internal conditions
macromolecule
a large molecule typically formed by the joining of smaller molecules
molecule
a chemical structure consisting of at least two atoms held together by a
chemical bond
organ
a structure formed of tissues operating together to perform a common
function
organ system
the higher level of organization that consists of functionally related
organs
organelle
a membrane-bound compartment or sac within a cell
organism
an individual living entity
phylogenetic tree
a diagram showing the evolutionary relationships among biological
species based on similarities and differences in genetic or physical
traits or both
population
all individuals within a species living within a specific area
prokaryote
a unicellular organism that lacks a nucleus or any other membrane-
bound organelle
tissue
a group of similar cells carrying out the same function
The Process of Science
By the end of this section, you will be able to:
e Describe the steps of the process of scientific inquiry and apply them
to specific examples.
e Distinguish between independent variables of interest, controlled
variables, and dependent variables.
e Describe controlled experiments and explain why they are desirable.
e Explain the basis of a double-blind experiment and how it can help
avoid bias.
Like geology, physics, and chemistry, biology is a science that gathers
knowledge about the natural world. Specifically, biology is the study of life.
The discoveries of biology are made by a community of researchers who
work individually and together using agreed-on methods. In this sense,
biology, like all sciences is a social enterprise like politics or the arts. The
methods of science include careful observation, record keeping, logical and
mathematical reasoning, experimentation, and submitting conclusions to the
scrutiny of others. Science also requires considerable imagination and
creativity; a well-designed experiment is commonly described as elegant, or
beautiful. Like politics, science has considerable practical implications and
some science is dedicated to practical applications, such as the prevention
of disease (see [link]). Other science proceeds largely motivated by
curiosity. Whatever its goal, there is no doubt that science, including
biology, has transformed human existence and will continue to do so.
Biologists may choose to study Escherichia
coli (E. coli), a bacterium that is a normal
resident of our digestive tracts but which is
also sometimes responsible for disease
outbreaks. In this micrograph, the bacterium
is visualized using a scanning electron
microscope and digital colorization. (credit:
Eric Erbe; digital colorization by
Christopher Pooley, USDA-ARS)
The Nature of Science
Biology is a science, but what exactly is science? What does the study of
biology share with other scientific disciplines? Science (from the Latin
scientia, meaning "knowledge") can be defined as knowledge about the
natural world.
Science is a very specific way of learning, or knowing, about the world.
The history of the past 500 years demonstrates that science is a very
powerful way of knowing about the world; it is largely responsible for the
technological revolutions that have taken place during this time. There are
however, areas of knowledge and human experience that the methods of
science cannot be applied to. These include such things as answering purely
moral questions, aesthetic questions, or what can be generally categorized
as spiritual questions. Science has cannot investigate these areas because
they are outside the realm of material phenomena, the phenomena of matter
and energy, and cannot be observed and measured.
The scientific method is a method of research with defined steps that
include experiments and careful observation. The steps of the scientific
method will be examined in detail later, but one of the most important
aspects of this method is the testing of hypotheses. A hypothesis is a
suggested explanation for an event, which can be tested. Hypotheses, or
tentative explanations, are generally produced within the context of a
scientific theory. A scientific theory is a generally accepted, thoroughly
tested and confirmed explanation for a set of observations or phenomena.
Scientific theory is the foundation of scientific knowledge. In addition, in
many scientific disciplines (less so in biology) there are scientific laws,
often expressed in mathematical formulas, which describe how elements of
nature will behave under certain specific conditions. There is not an
evolution of hypotheses through theories to laws as if they represented
some increase in certainty about the world. Hypotheses are the day-to-day
material that scientists work with and they are developed within the context
of theories. Laws are concise descriptions of parts of the world that are
amenable to formulaic or mathematical description.
Scientific Inquiry
One thing is common to all forms of science: an ultimate goal “to know.”
Curiosity and inquiry are the driving forces for the development of science.
Scientists seek to understand the world and the way it operates. There are
two main pathways of scientific study: descriptive science and hypothesis-
based science. Descriptive (or discovery) science aims to observe, explore,
and discover, while hypothesis-based science begins with a specific
question or problem and a potential answer or solution that can be tested.
The boundary between these two forms of study is often blurred, because
most scientific endeavors combine both approaches. Observations lead to
questions, questions lead to forming a hypothesis as a possible answer to
those questions, and then the hypothesis is tested. Thus, descriptive science
and hypothesis-based science are in continuous dialogue.
Hypothesis Testing
Biologists study the living world by posing questions about it and seeking
science-based responses. This approach is common to other sciences as well
and is often referred to as the scientific method. The scientific method was
used even in ancient times, but it was first documented by England’s Sir
Francis Bacon (1561-1626) ({link]). The scientific method is not
exclusively used by biologists but can be applied to almost anything as a
logical problem-solving method.
Sir Francis Bacon
is credited with
being the first to
document the
scientific method.
The scientific process typically starts with an observation (often a problem
to be solved) that leads to a question. Let’s think about a simple problem
that starts with an observation and apply the scientific method to solve the
problem. One Monday morning, a student arrives at class and quickly
discovers that the classroom is too warm. That is an observation that also
describes a problem: the classroom is too warm. The student then asks a
question: “Why is the classroom so warm?”
Recall that a hypothesis is a suggested explanation that can be tested. To
solve a problem, several hypotheses may be proposed. For example, one
hypothesis might be, “The classroom is warm because no one turned on the
air conditioning.” But there could be other responses to the question, and
therefore other hypotheses may be proposed. A second hypothesis might be,
“The classroom is warm because there is a power failure, and so the air
conditioning doesn’t work.”
Once a hypothesis has been selected, a prediction may be made. A
prediction is similar to a hypothesis but it typically has the format “If...
then ....” For example, the prediction for the first hypothesis might be, “If
the student turns on the air conditioning, then the classroom will no longer
be too warm. Notice that the portion of the statement after the word "then"
indicates what will be observed if the hypothesis is correct.
A hypothesis must be testable to ensure that it is valid. For example, a
hypothesis that depends on what a bear thinks is not testable, because it can
never be known what a bear thinks. It should also be falsifiable, meaning
that it can be shown to be incorrect by experimental results. An example of
an unfalsifiable hypothesis is “Botticelli’s Birth of Venus is beautiful.”
There is no experiment that might show this statement to be false. To test a
hypothesis, a researcher will conduct one or more experiments designed to
eliminate one or more of the hypotheses. This is important. A hypothesis
can be shown to be incorrect, or eliminated, but it can never be proven.
Science does not deal in proofs like mathematics. If an experiment fails to
show a hypothesis is incorrect, then we find support for that explanation,
but this is not to say that down the road a better explanation will not be
found, or a more carefully designed experiment will be found to falsify the
hypothesis.
Each experiment will have one or more variables and one or more controls.
A variable is any part of the experiment that can vary or change during the
experiment. There are three types of variables we will discuss: Independent
variable(s) of interest, independent variables not of interest (i.e. controlled
variables), and dependent variables. Typically, basic experiments will only
have one independent variable of interest (i.e. the factor that is being
changed in a deliberate manner to determine if it has an impact on the
dependent variable). The dependent variable is the one being measured. For
example, if an experiment is designed to test the effects of three different
brands of plant fertilizer on plant growth, the independent variable of
interest is the brand of fertilizer and the dependent variable is plant growth.
It is important to realize that independent variables other than the brand of
fertilizer could affect plant growth: soil moisture content, amount of
sunlight, temperature, etc. In order to prevent these variables from
impacting the results, they are "controlled", meaning they are not allowed to
change during the experiment. In other words, plants treated with all three
brands of fertilizer should have the same amounts of water and sunlight to
prevent these variables from interacting with the independent variable of
interest. A control group is a part of the experiment that does not change
and provides a baseline of comparison to determine the effect of the
independent variable of interest on the dependent variable. Look for the
variables and controls in the example that follows. An experiment is
conducted to test the hypothesis that phosphate limits the growth of algae in
freshwater ponds. A series of artificial ponds are filled with water and half
of them are treated by adding phosphate each week, while the other half are
treated by adding a salt that is known not to be used by algae. The
independent variable of interest here is the phosphate (or lack of
phosphate), the experimental or treatment cases are the ponds with added
phosphate and the control ponds are those with something inert added, such
as the salt. Just adding something is also a control against the possibility
that adding extra matter to the pond has an effect. If the treated ponds show
lesser growth of algae, then we have found support for our hypothesis. If
they do not, then we reject our hypothesis. Be aware that rejecting one
hypothesis does not determine whether or not the other hypotheses can be
accepted; it simply eliminates one hypothesis that is not valid ([link]).
Using the scientific method, the hypotheses that are inconsistent with
experimental data are rejected. In human drug trials, it is common for the
control group to be given a placebo (i.e. "sugar pill") so that individuals
both groups (experimental and control) are taking a pill. Otherwise, the act
of taking a pill would be a variable that wasn't controlled. It is also common
for neither the subjects nor the researchers directly working with them to
know which group is receiving the placebo. This feature of experiments,
called a double-blind design, is included to prevent any bias from
influencing the results.
Note:
Art Connection
Make an observation
Ask a question
Form a hypothesis that
answers the question
Make a prediction based
on the hypothesis
Do an experiment
to test the prediction
Analyze the results
Hypothesis is
SUPPORTED
Report results
Hypothesis is
NOT SUPPORTED
The scientific method is a series
of defined steps that include
experiments and careful
observation. If a hypothesis is
not supported by data, a new
hypothesis can be proposed.
In practice, the scientific method is not as rigid and structured as it might at
first appear. Sometimes an experiment leads to conclusions that favor a
change in approach; often, an experiment brings entirely new scientific
questions to the puzzle. Many times, science does not operate in a linear
fashion; instead, scientists continually draw inferences and make
generalizations, finding patterns as their research proceeds. Scientific
reasoning is more complex than the scientific method alone suggests.
Reporting Scientific Work
Whether scientific research is basic science or applied science, scientists
must share their findings for other researchers to expand and build upon
their discoveries. Communication and collaboration within and between sub
disciplines of science are key to the advancement of knowledge in science.
For this reason, an important aspect of a scientist’s work is disseminating
results and communicating with peers. Scientists can share results by
presenting them at a scientific meeting or conference, but this approach can
reach only the limited few who are present. Instead, most scientists present
their results in peer-reviewed articles that are published in scientific
journals. Peer-reviewed articles are scientific papers that are reviewed,
usually anonymously by a scientist’s colleagues, or peers. These colleagues
are qualified individuals, often experts in the same research area, who judge
whether or not the scientist’s work is suitable for publication. The process
of peer review helps to ensure that the research described in a scientific
paper or grant proposal is original, significant, logical, and thorough. Grant
proposals, which are requests for research funding, are also subject to peer
review. Scientists publish their work so other scientists can reproduce their
experiments under similar or different conditions to expand on the findings.
The experimental results must be consistent with the findings of other
scientists.
There are many journals and the popular press that do not use a peer-review
system. A large number of online open-access journals, journals with
articles available without cost, are now available many of which use
rigorous peer-review systems, but some of which do not. Results of any
studies published in these forums without peer review are not reliable and
should not form the basis for other scientific work. In one exception,
journals may allow a researcher to cite a personal communication from
another researcher about unpublished results with the cited author’s
permission.
Section Summary
A hypothesis is a tentative explanation for an observation. A scientific
theory is a well-tested and consistently verified explanation for a set of
observations or phenomena. A scientific law is a description, often in the
form of a mathematical formula, of the behavior of an aspect of nature
under certain circumstances. The common thread throughout scientific
research is the use of the scientific method. Scientists present their results in
peer-reviewed scientific papers published in scientific journals.
Art Connections
Exercise:
Problem:
[link] In the example below, the scientific method is used to solve an
everyday problem. Which part in the example below is the hypothesis?
Which is the prediction? Based on the results of the experiment, is the
hypothesis supported? If it is not supported, propose some alternative
hypotheses.
1. My toaster doesn’t toast my bread.
2. Why doesn’t my toaster work?
3. There is something wrong with the electrical outlet.
4. If something is wrong with the outlet, my coffeemaker also won’t
work when plugged into it.
5. I plug my coffeemaker into the outlet.
6. My coffeemaker works.
Solution:
[link] The hypothesis is #3 (there is something wrong with the
electrical outlet), and the prediction is #4 (if something is wrong with
the outlet, then the coffeemaker also won’t work when plugged into
the outlet). The original hypothesis is not supported, as the coffee
maker works when plugged into the outlet. Alternative hypotheses
may include (1) the toaster might be broken or (2) the toaster wasn’t
turned on.
Multiple Choice
Exercise:
Problem:
A suggested and testable explanation for an event is called a
a. hypothesis
b. variable
c. theory
d. control
Solution:
A
Exercise:
Problem:
Which of the following statements correct describes a double-blind
experiment?
a. Both the test subjects and the researchers in direct contact with
them know who is in the experimental group and who is in the
control group.
b. The test subjects but not the researchers in direct contact with
them know who is in the experimental group and who is in the
control group.
c. The researchers in direct contact with the test subjects, but not the
test subjects, know who is in the experimental group and who is
in the control group.
d. Neither the test subjects or the researchers in direct contact with
them know who is in the experimental group and who is in the
control group.
Solution:
D
Free Response
Exercise:
Problem:
A research group is testing a new disease vaccine on mice. Which
group (experimental or control) should receive the placebo? Explain
the rationale for your response.
Solution:
The control group should receive the placebo, or injection without the
active components of the vaccine. The response in the control group
will provide a baseline of comparison for interpreting the results of the
experimental group.
Glossary
applied science
a form of science that solves real-world problems
basic science
science that seeks to expand knowledge regardless of the short-term
application of that knowledge
control
a part of an experiment that does not change during the experiment
deductive reasoning
a form of logical thinking that uses a general statement to forecast
specific results
descriptive science
a form of science that aims to observe, explore, and find things out
falsifiable
able to be disproven by experimental results
hypothesis
a suggested explanation for an event, which can be tested
hypothesis-based science
a form of science that begins with a specific explanation that is then
tested
inductive reasoning
a form of logical thinking that uses related observations to arrive at a
general conclusion
life science
a field of science, such as biology, that studies living things
natural science
a field of science that studies the physical world, its phenomena, and
processes
peer-reviewed article
a scientific report that is reviewed by a scientist’s colleagues before
publication
physical science
a field of science, such as astronomy, physics, and chemistry, that
studies nonliving matter
science
knowledge that covers general truths or the operation of general laws,
especially when acquired and tested by the scientific method
scientific law
a description, often in the form of a mathematical formula, for the
behavior of some aspect of nature under certain specific conditions
scientific method
a method of research with defined steps that include experiments and
careful observation
scientific theory
a thoroughly tested and confirmed explanation for observations or
phenomena
variable
a part of an experiment that can vary or change
Introduction
class="introduction"
Foods such as bread,
fruit, and cheese are
rich sources of
biological
macromolecules,
such as starch, fiber,
triglycerides, and
polypeptides/proteins
. (credit: modification
of work by Bengt
Nyman)
The elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus
are the key building blocks of the chemicals found in living things. They
form the carbohydrates, nucleic acids, proteins, and lipids (all of which will
be defined later in this chapter) that are the fundamental molecular
components of all organisms. In this chapter, we will discuss these
important building blocks and learn how the unique properties of the atoms
of different elements affect their interactions with other atoms to form the
molecules of life.
Food provides an organism with nutrients—the matter it needs to survive.
Many of these critical nutrients come in the form of biological
macromolecules, or large molecules necessary for life. These
macromolecules are built from different combinations of smaller organic
molecules. What specific types of biological macromolecules do living
things require? How are these molecules formed? What functions do they
serve? In this chapter, we will explore these questions.
The Building Blocks of Molecules
By the end of this section, you will be able to:
e Describe matter and elements
e Describe the interrelationship between protons, neutrons, and
electrons, and the ways in which electrons can be donated or shared
between atoms
At its most fundamental level, life is made up of matter. Matter occupies
space and has mass. All matter is composed of elements, substances that
cannot be broken down or transformed chemically into other substances.
Each element is made of atoms, each with a constant number of protons and
unique properties. A total of 118 elements have been defined; however,
only 92 occur naturally, and fewer than 30 are found in living cells. The
remaining 26 elements are unstable and, therefore, do not exist for very
long or are theoretical and have yet to be detected.
Each element is designated by its chemical symbol (such as H, N, O, C, and
Na), and possesses unique properties. These unique properties allow
elements to combine and to bond with each other in specific ways.
Atoms
An atom is the smallest component of an element that retains all of the
chemical properties of that element. For example, one hydrogen atom has
all of the properties of the element hydrogen, such as it exists as a gas at
room temperature, and it bonds with oxygen to create a water molecule.
Hydrogen atoms cannot be broken down into anything smaller while still
retaining the properties of hydrogen. If a hydrogen atom were broken down
into subatomic particles (i.e. protons, neutrons, and electrons), it would no
longer have the properties of hydrogen.
At the most basic level, all organisms are made of a combination of
elements. They contain atoms that combine together to form molecules. In
multicellular organisms, such as animals, molecules can interact to form
cells that combine to form tissues, which make up organs. These
combinations continue until entire multicellular organisms are formed.
All atoms contain protons, electrons, and neutrons ((link]). The only
exception is hydrogen (H), which is made of one proton and one electron. A
proton is a positively charged particle that resides in the nucleus (the core
of the atom) of an atom and has a mass of 1 and a charge of +1. An
electron is a negatively charged particle that travels in the space around the
nucleus. In other words, it resides outside of the nucleus. It has a negligible
mass (i.e. considered to be zero compared to protons and neutrons) and has
a charge of —1.
\
\ '
A) — Nucleus
} |
f /
V4
Electrons
Atoms are made up of protons
and neutrons located within the
nucleus, and electrons
surrounding the nucleus.
Neutrons, like protons, reside in the nucleus of an atom. They have a mass
of 1 and no charge. The positive (protons) and negative (electrons) charges
balance each other in a neutral atom, which has a net zero charge.
Because protons and neutrons each have a mass of 1, the mass of an atom is
equal to the combined number of protons and neutrons of that atom. The
number of electrons does not factor into the overall mass, because their
mass is so small.
As stated earlier, each element has its own unique properties. Each contains
a different number of protons and neutrons, giving it its own atomic number
and mass number. The atomic number of an element is equal to the
number of protons that element contains. The mass number, or atomic
mass, is the number of protons plus the number of neutrons of that element.
Therefore, it is possible to determine the number of neutrons by subtracting
the atomic number from the mass number. For example, the element
phosphorus (P) has an atomic number of 15 and a mass number of 31.
Therefore, an atom of phosphorus has 15 protons, 15 electrons, and 16
neutrons (31-15 = 16).
These numbers provide information about the elements and how they will
react when combined. Different elements have different melting and boiling
points, and are in different states (liquid, solid, or gas) at room temperature.
They also combine in different ways. Some form specific types of bonds,
whereas others do not. How they combine is based on the number of
electrons present. Because of these characteristics, the elements are
arranged into the periodic table of elements, a chart of the elements that
includes the atomic number and relative atomic mass of each element. The
periodic table also provides key information about the properties of
elements ({link])—often indicated by color-coding. The arrangement of the
table also shows how the electrons in each element are organized and
provides important details about how atoms will react with each other to
form molecules.
Isotopes are different forms of the same element that have the same number
of protons, but a different number of neutrons. Some elements, such as
carbon, potassium, and uranium, have naturally occurring isotopes. Carbon-
12, the most common isotope of carbon, contains six protons and six
neutrons. Therefore, it has a mass number of 12 (six protons and six
neutrons) and an atomic number of 6 (which makes it carbon). Carbon-14
contains six protons and eight neutrons. Therefore, it has a mass number of
14 (six protons and eight neutrons) and an atomic number of 6, meaning it
is still the element carbon. These two alternate forms of carbon are isotopes.
Some isotopes are unstable and will lose protons, other subatomic particles,
or energy to form more stable elements. These are called radioactive
isotopes or radioisotopes.
Note:
Art Connection
Periodic Table of the Elements
~afs P s e [i] i
ARB A ee eee eee
; [| Other non-metals |_| Noble gases
Number ; :
Symbol [_] Alkali metals {| Lanthanides
1.01 Relative [| Transition metals [_] Actinides
Name Hydrogen Atomic Mass [D) Other metals [_] Unknown
[_] Alkaline earth metals chemical
properties
|_| Halogens
Arranged in columns and rows based on the characteristics of
the elements, the periodic table provides key information about
the elements and how they might interact with each other to
form molecules. Most periodic tables provide a key or legend
to the information they contain.
Note:
Evolution in Action
Carbon Dating
Carbon-14 ('4C) is a naturally occurring radioisotope that is created in the
atmosphere by cosmic rays. This is a continuous process, so more C is
always being created. As a living organism develops, the relative level of
'4C in its body is equal to the concentration of '4C in the atmosphere.
When an organism dies, it is no longer ingesting '4C, so the ratio will
decline. '4C decays to !“N by a process called beta decay; it gives off
energy in this slow process.
After approximately 5,730 years, only one-half of the starting
concentration of !4C will have been converted to !4N. The time it takes for
half of the original concentration of an isotope to decay to its more stable
form is called its half-life. Because the half-life of !4C is long, it is used to
age formerly living objects, such as fossils. Using the ratio of the !4C
concentration found in an object to the amount of !4C detected in the
atmosphere, the amount of the isotope that has not yet decayed can be
determined. Based on this amount, the age of the fossil can be calculated to
about 50,000 years ({link]). Isotopes with longer half-lives, such as
potassium-40, are used to calculate the ages of older fossils. Through the
use of carbon dating, scientists can reconstruct the ecology and
biogeography of organisms living within the past 50,000 years.
The age of remains that contain
carbon and are less than about 50,000
years old, such as this pygmy
mammoth, can be determined using
carbon dating. (credit: Bill
Faulkner/NPS)
Note:
Concept in Action
[=] qa
= openstax See
tare E
Oe at
To learn more about atoms and isotopes, and how you can tell one isotope
from another, visit this site and run the simulation.
Chemical Bonds
How elements interact with one another depends on how their electrons are
arranged and how many openings for electrons exist at the outermost region
where electrons are present in an atom. Electrons exist at energy levels that
form shells around the nucleus. The closest shell can hold up to two
electrons. The closest shell to the nucleus is always filled first, before any
other shell can be filled. Hydrogen has one electron; therefore, it has only
one spot occupied within the lowest shell. Helium has two electrons;
therefore, it can completely fill the lowest shell with its two electrons. If
you look at the periodic table, you will see that hydrogen and helium are the
only two elements in the first row. This is because they only have electrons
in their first shell. Hydrogen and helium are the only two elements that have
the lowest shell and no other shells.
The second and third energy levels can hold up to eight electrons. The eight
electrons are arranged in four pairs and one position in each pair is filled
with an electron before any pairs are completed.
Looking at the periodic table again ({link]), you will notice that there are
seven rows. These rows correspond to the number of shells that the
elements within that row have. The elements within a particular row have
increasing numbers of electrons as the columns proceed from left to right.
Although each element has the same number of shells, not all of the shells
are completely filled with electrons. If you look at the second row of the
periodic table, you will find lithium (Li), beryllium (Be), boron (B), carbon
(C), nitrogen (N), oxygen (O), fluorine (F), and neon (Ne). These all have
electrons that occupy only the first and second shells. Lithium has only one
electron in its outermost shell, beryllium has two electrons, boron has three,
and so on, until the entire shell is filled with eight electrons, as is the case
with neon.
Not all elements have enough electrons to fill their outermost shells, but an
atom is at its most stable when all of the electron positions in the outermost
shell are filled. Because of these vacancies in the outermost shells, we see
the formation of chemical bonds, or interactions between two or more of
the same or different elements that result in the formation of molecules. To
achieve greater stability, atoms will tend to completely fill their outer shells
and will bond with other elements to accomplish this goal by sharing
electrons, accepting electrons from another atom, or donating electrons to
another atom. Because the outermost shells of the elements with low atomic
numbers (up to calcium, with atomic number 20) can hold eight electrons,
this is referred to as the octet rule. An element can donate, accept, or share
electrons with other elements to fill its outer shell and satisfy the octet rule.
When an atom does not contain equal numbers of protons and electrons, it
is called an ion. Because the number of electrons does not equal the number
of protons, each ion has a net charge. Positive ions are formed by losing
electrons and are called cations. Negative ions are formed by gaining
electrons and are called anions.
For example, sodium only has one electron in its outermost shell. It takes
less energy for sodium to donate that one electron than it does to accept
seven more electrons to fill the outer shell. If sodium loses an electron, it
now has 11 protons and only 10 electrons, leaving it with an overall charge
of +1. It is now called a sodium ion.
The chlorine atom has seven electrons in its outer shell. Again, it is more
energy-efficient for chlorine to gain one electron than to lose seven.
Therefore, it tends to gain an electron to create an ion with 17 protons and
18 electrons, giving it a net negative (—1) charge. It is now called a chloride
ion. This movement of electrons from one element to another is referred to
as electron transfer. As [link] illustrates, a sodium atom (Na) only has one
electron in its outermost shell, whereas a chlorine atom (Cl) has seven
electrons in its outermost shell. A sodium atom will donate its one electron
to empty its shell, and a chlorine atom will accept that electron to fill its
shell, becoming chloride. Both ions now satisfy the octet rule and have
complete outermost shells. Because the number of electrons is no longer
equal to the number of protons, each is now an ion and has a +1 (sodium) or
—1 (chloride) charge.
Elements tend to fill their outermost
Shells with electrons. To do this, they
can either donate or accept electrons
from other elements.
Ionic Bonds
We will study three types of bonds or interactions: ionic, covalent, and
hydrogen bonds. When an element donates an electron from its outer shell,
as in the sodium atom example above, a positive ion is formed. The element
accepting the electron is now negatively charged. Because positive and
negative charges attract, these ions stay together and form an ionic bond, or
a bond between ions. The elements bond together with the electron from
one element staying predominantly with the other element. When Na™ and
CI ions combine to produce NaCl, an electron from a sodium atom stays
with the other seven from the chlorine atom, and the sodium and chloride
ions attract each other in a lattice of ions with a net zero charge.
Covalent Bonds
Another type of chemical bond between two or more atoms is a covalent
bond. These bonds form when an electron is shared between two elements
and are the strongest and most common form of chemical bond in living
organisms. Covalent bonds form between the elements that make up the
biological molecules in our cells. Unlike ionic bonds, covalent bonds do not
dissociate (i.e. separate) in water.
The hydrogen and oxygen atoms that combine to form water molecules are
bound together by covalent bonds. The electron from the hydrogen atom
divides its time between the outer shell of the hydrogen atom and the
incomplete outer shell of the oxygen atom. To completely fill the outer shell
of an oxygen atom, two electrons from two hydrogen atoms are needed,
hence the subscript “2” in H)O. The electrons are shared between the
atoms, dividing their time between them to “fill” the outer shell of each.
This sharing is a lower energy state for all of the atoms involved than if
they existed without their outer shells filled.
There are two types of covalent bonds: polar and nonpolar. Nonpolar
covalent bonds form between two atoms of the same element or between
different elements that share the electrons equally. For example, an oxygen
atom can bond with another oxygen atom to fill their outer shells. This
association is nonpolar because the electrons will be equally distributed
between each oxygen atom. Two covalent bonds form between the two
oxygen atoms because oxygen requires two shared electrons to fill its
outermost shell. Nitrogen atoms will form three covalent bonds (also called
triple covalent) between two atoms of nitrogen because each nitrogen atom
needs three electrons to fill its outermost shell. Another example of a
nonpolar covalent bond is found in the methane (CHy) molecule. The
carbon atom has four electrons in its outermost shell and needs four more to
fill it. It gets these four from four hydrogen atoms, each atom providing
one. These elements all share the electrons equally, creating four nonpolar
covalent bonds ((link]).
In a polar covalent bond, the electrons shared by the atoms spend more
time closer to one nucleus than to the other nucleus. Because of the unequal
distribution of electrons between the different nuclei, a slightly positive
(5+) or slightly negative (S—) charge develops. The covalent bonds between
hydrogen and oxygen atoms in water are polar covalent bonds. The shared
electrons spend more time near the oxygen nucleus, giving it a small
negative charge, than they spend near the hydrogen nuclei, giving these
molecules a small positive charge.
Polar covalent bond Nonpolar covalent bond Nonpolar covalent double bond
‘ ) v OH)
s-
=— Single bond
= Double bond
The water molecule (left) depicts a polar bond with a
slightly positive charge on the hydrogen atoms and a
slightly negative charge on the oxygen. Examples of
nonpolar bonds include methane (middle) and oxygen
(right).
Hydrogen Bonds
Ionic and covalent bonds are strong bonds that require considerable energy
to break. However, not all bonds between elements are ionic or covalent
bonds. Weaker bonds can also form. These are attractions that occur
between positive and negative charges that do not require much energy to
break. An example of relatively weak bonds that occur frequently is
hydrogen bonds. This bond gives rise to the unique properties of water and
the unique structures of DNA and proteins.
When polar covalent bonds containing a hydrogen atom form, the hydrogen
atom in that bond has a slightly positive charge. This is because the shared
electron is pulled more strongly toward the other element and away from
the hydrogen nucleus. Because the hydrogen atom is slightly positive (6+),
it will be attracted to neighboring negative partial charges (6—). When this
happens, a weak interaction occurs between the 6+ charge of the hydrogen
atom of one molecule and the 6— charge of the other molecule. This
interaction is called a hydrogen bond. This type of bond is common; for
example, the liquid nature of water is caused by the hydrogen bonds
between water molecules ([link]). Hydrogen bonds give water the unique
properties that sustain life. If it were not for hydrogen bonding, water would
be a gas rather than a liquid at room temperature.
Covalent bond
i, (
Hydrogen bond
Hydrogen bonds form between
slightly positive (6+) and slightly
negative (d—) charges of polar
covalent molecules, such as water.
Hydrogen bonds can form between different molecules and they do not
always have to include a water molecule. Hydrogen atoms in polar bonds
within any molecule can form bonds with other adjacent molecules. For
example, hydrogen bonds hold together two long strands of DNA to give
the DNA molecule its characteristic double-helix structure. Hydrogen bonds
are also responsible for some of the three-dimensional structure of proteins.
Section Summary
Matter is anything that occupies space and has mass. It is made up of atoms
of different elements. All of the 92 elements that occur naturally have
unique qualities that allow them to combine in various ways to create
compounds or molecules. Atoms, which consist of protons, neutrons, and
electrons, are the smallest units of an element that retain all of the
properties of that element. Electrons can be donated or shared between
atoms to create bonds, including ionic, covalent, and hydrogen bonds.
Art Connections
Exercise:
Problem:
[link] How many neutrons do (K) potassium-39 and potassium-40
have, respectively?
Solution:
[link] Potassium-39 has twenty neutrons. Potassium-40 has twenty one
neutrons.
Multiple Choice
Exercise:
Problem:
Magnesium has an atomic number of 12. Which of the following
statements is true of a neutral magnesium atom?
a. It has 12 protons, 12 electrons, and 12 neutrons.
b. It has 12 protons, 12 electrons, and six neutrons.
c. It has six protons, six electrons, and no neutrons.
d. It has six protons, six electrons, and six neutrons.
Solution:
A
Exercise:
Problem: Which type of bond represents a weak chemical bond?
a. hydrogen bond
b. ionic bond
c. covalent bond
d. polar covalent bond
Solution:
A
Exercise:
Problem:
An isotope of sodium (Na) has a mass number of 22. How many
neutrons does it have?
a. 11
b..12
CZ2
d. 44
Solution:
A
Free Response
Exercise:
Problem: Why are hydrogen bonds necessary for cells?
Solution:
Hydrogen bonds form weak associations between different molecules.
They provide the structure and shape necessary for proteins and DNA
within cells so that they function properly. Hydrogen bonds also give
water its unique properties, which are necessary for life.
Glossary
anion
a negative ion formed by gaining electrons
atomic number
the number of protons in an atom
cation
a positive ion formed by losing electrons
chemical bond
an interaction between two or more of the same or different elements
that results in the formation of molecules
covalent bond
a type of strong bond between two or more of the same or different
elements; forms when electrons are shared between elements
electron
a negatively charged particle that resides outside of the nucleus in the
electron orbital; lacks functional mass and has a charge of —1
electron transfer
the movement of electrons from one element to another
element
one of 118 unique substances that cannot be broken down into smaller
substances and retain the characteristic of that substance; each element
has a specified number of protons and unique properties
hydrogen bond
a weak bond between partially positively charged hydrogen atoms and
partially negatively charged elements or molecules
ion
an atom or compound that does not contain equal numbers of protons
and electrons, and therefore has a net charge
ionic bond
a chemical bond that forms between ions of opposite charges
isotope
one or more forms of an element that have different numbers of
neutrons
mass number
the number of protons plus neutrons in an atom
matter
anything that has mass and occupies space
neutron
a particle with no charge that resides in the nucleus of an atom; has a
mass of 1
nonpolar covalent bond
a type of covalent bond that forms between atoms when electrons are
shared equally between atoms, resulting in no regions with partial
charges as in polar covalent bonds
nucleus
(chemistry) the dense center of an atom made up of protons and
(except in the case of a hydrogen atom) neutrons
octet rule
states that the outermost shell of an element with a low atomic number
can hold eight electrons
periodic table of elements
an organizational chart of elements, indicating the atomic number and
mass number of each element; also provides key information about the
properties of elements
polar covalent bond
a type of covalent bond in which electrons are pulled toward one atom
and away from another, resulting in slightly positive and slightly
negative charged regions of the molecule
proton
a positively charged particle that resides in the nucleus of an atom; has
a mass of 1 and a charge of +1
radioactive isotope
an isotope that spontaneously emits particles or energy to form a more
stable element
van der Waals interaction
a weak attraction or interaction between molecules caused by slightly
positively charged or slightly negatively charged atoms
The Chemical and Physical Properties of Water
By the end of this section, you will be able to:
e Describe the properties of water that are critical to maintaining life
e Distinguish between hydrophilic and hydrophobic molecules
e Explain why water is a good solvent for many solutes
e Explain the pH scale using the specified terms; indicate how much
more (or less) acidic one substance is compared to another using the
PH scale
Do you ever wonder why scientists spend time looking for water on other
planets? It is because water is essential to life; even minute traces of it on
another planet can indicate that life could or did exist on that planet. Water
is one of the more abundant molecules in living cells and the one most
critical to life as we know it. Approximately 60—70 percent of your body is
made up of water. Without it, life simply would not exist.
Water Is Polar
The hydrogen and oxygen atoms within water molecules form polar
covalent bonds. The shared electrons spend more time associated with the
oxygen atom than they do with hydrogen atoms. There is no overall charge
to a water molecule, but there is a slight positive charge on each hydrogen
atom and a slight negative charge on the oxygen atom. Because of these
charges, the slightly positive hydrogen atoms repel each other. Each water
molecule attracts other water molecules because of the positive and
negative charges in the different parts of the molecule. Water also attracts
other polar molecules (such as sugars), forming hydrogen bonds. When a
substance readily forms hydrogen bonds with water, it can dissolve in water
and is referred to as hydrophilic (“water-loving”). Hydrogen bonds are not
readily formed with nonpolar substances like oils and fats ((link]). These
nonpolar compounds are hydrophobic (“water-fearing”) and will not
dissolve in water.
As this macroscopic image of
oil and water show, oil is a
nonpolar compound and,
hence, will not dissolve in
water. Oil and water do not
mix. (credit: Gautam Dogra)
Water Stabilizes Temperature
The hydrogen bonds in water allow it to absorb and release heat energy
more slowly than many other substances. Temperature is a measure of the
motion (kinetic energy) of molecules. As the motion increases, energy is
higher and thus temperature is higher. Water absorbs a great deal of energy
before its temperature rises. Increased energy disrupts the hydrogen bonds
between water molecules. Because these bonds can be created and disrupted
rapidly, water absorbs an increase in energy and temperature changes only
minimally. This means that water moderates temperature changes within
organisms and in their environments. As energy input continues, the
balance between hydrogen-bond formation and destruction swings toward
the destruction side. More bonds are broken than are formed. This process
results in the release of individual water molecules at the surface of the
liquid (such as a body of water, the leaves of a plant, or the skin of an
organism) in a process called evaporation. Evaporation of sweat, which is
90 percent water, allows for cooling of an organism, because breaking
hydrogen bonds requires an input of energy and takes heat away from the
body.
Water Is an Excellent Solvent
Because water is polar, with slight positive and negative charges, ionic
compounds and polar molecules can readily dissolve in it. Water is,
therefore, what is referred to as a solvent—a substance capable of
dissolving another substance, referred to as the solute, in order to form a
solution. The charged particles will form hydrogen bonds with a
surrounding layer of water molecules. This is referred to as a sphere of
hydration and serves to keep the particles separated or dispersed in the
water. In the case of table salt (NaCl) mixed in water ([link]), the sodium
and chloride ions separate, or dissociate, in the water, and spheres of
hydration are formed around the ions. A positively charged sodium ion is
surrounded by the partially negative charges of oxygen atoms in water
molecules. A negatively charged chloride ion is surrounded by the partially
positive charges of hydrogen atoms in water molecules. The polarity of the
water molecule makes it an effective solvent and is important in its many
roles in living systems.
A single water molecule
with partial charges
3-
When table salt (NaCl) is mixed in water, spheres
of hydration form around the ions.
Buffers, pH, Acids, and Bases
The pH of a solution is a measure of its acidity or alkalinity. You have
probably used litmus paper, paper that has been treated with a natural
water-soluble dye so it can be used as a pH indicator, to test how much acid
or base (alkalinity) exists in a solution. You might have even used some to
make sure the water in an outdoor swimming pool is properly treated. In
both cases, this pH test measures the amount of hydrogen ions that exists in
a given solution. High concentrations of hydrogen ions yield a low pH,
whereas low levels of hydrogen ions result in a high pH. The overall
concentration of hydrogen ions is inversely related to its pH and can be
measured on the pH scale ((link]). Therefore, the more hydrogen ions
present, the lower the pH; conversely, the fewer hydrogen ions, the higher
the pH.
The pH scale ranges from 0 to 14. A change of one unit on the pH scale
represents a change in the concentration of hydrogen ions by a factor of 10,
a change in two units represents a change in the concentration of hydrogen
ions by a factor of 100. Thus, small changes in pH represent large changes
in the concentrations of hydrogen ions. Pure water is neutral. It is neither
acidic nor basic, and has a pH of 7.0. Anything below 7.0 (ranging from 0.0
to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline. The
blood in your veins is slightly alkaline (pH = 7.4). The environment in your
stomach is highly acidic (pH = 1 to 2). Orange juice is mildly acidic (pH =
approximately 3.5), whereas baking soda is basic (pH = 9.0).
Bleach
Soapy water
Ammonia solution
Milk of magnesia
Baking soda
Seawater
Distilled water
Urine
5 Black coffee
4 Tomato juice
3 Orange juice
2 Lemon juice
1 Gastric acid
0
The pH scale measures the amount
of hydrogen ions (H*) ina
substance. (credit: modification of
work by Edward Stevens)
Acids are substances that provide hydrogen ions (H*) and lower pH,
whereas bases provide hydroxide ions (OH _) and raise pH. The stronger the
acid, the more readily it donates H*. For example, hydrochloric acid and
lemon juice are very acidic and readily give up H” when added to water.
Conversely, bases are those substances that readily donate OH. The OH
ions combine with H™ to produce water, which raises a substance’s pH.
Sodium hydroxide and many household cleaners are very alkaline and give
up OH rapidly when placed in water, thereby raising the pH.
Most cells in our bodies operate within a very narrow window of the pH
scale, typically ranging only from 7.2 to 7.6. If the pH of the body is outside
of this range, the respiratory system malfunctions, as do other organs in the
body. Cells no longer function properly, and proteins will break down.
Deviation outside of the pH range can induce coma or even cause death.
So how is it that we can ingest or inhale acidic or basic substances and not
die? Buffers are the key. Buffers readily absorb excess H* or OH’, keeping
the pH of the body carefully maintained in the aforementioned narrow
range. Carbon dioxide is part of a prominent buffer system in the human
body; it keeps the pH within the proper range. This buffer system involves
carbonic acid (H»CO3) and bicarbonate (HCO; ) anion. If too much H*
enters the body, bicarbonate will combine with the H* to create carbonic
acid and limit the decrease in pH. Likewise, if too much OH7 is introduced
into the system, carbonic acid will rapidly dissociate into bicarbonate and
H"* ions. The H* ions can combine with the OH™ ions, limiting the increase
in pH. While carbonic acid is an important product in this reaction, its
presence is fleeting because the carbonic acid is released from the body as
carbon dioxide gas each time we breathe. Without this buffer system, the
PH in our bodies would fluctuate too much and we would fail to survive.
Section Summary
Water has many properties that are critical to maintaining life. It is polar,
allowing for the formation of hydrogen bonds, which allow ions and other
polar molecules to dissolve in water. Therefore, water is an excellent
solvent. The hydrogen bonds between water molecules give water the
ability to hold heat better than many other substances. As the temperature
rises, the hydrogen bonds between water continually break and reform,
allowing for the overall temperature to remain stable, although increased
energy is added to the system. All of these unique properties of water are
important in the chemistry of living organisms.
The pH of a solution is a measure of the concentration of hydrogen ions in
the solution. A solution with a high number of hydrogen ions is acidic and
has a low pH value. A solution with a high number of hydroxide ions is
basic and has a high pH value. The pH scale ranges from 0 to 14, with a pH
of 7 being neutral. Buffers are solutions that moderate pH changes when an
acid or base is added to the buffer system. Buffers are important in
biological systems because of their ability to maintain constant pH
conditions.
Multiple Choice
Exercise:
Problem: Which of the following statements is FALSE?
a. Water is polar.
b. Water stabilizes temperature.
c. Water is essential for life.
d. Water is the most abundant atom in Earth’s atmosphere.
Solution:
D
Exercise:
Problem:
Using a pH meter, you find the pH of an unknown solution to be 8.0.
How would you describe this solution?
a. weakly acidic
b. strongly acidic
c. weakly basic
d. strongly basic
Solution:
C
Exercise:
Problem:
The pH of lemon juice is about 2.0, whereas tomato juice's pH is about
4. Therefore, the concentration of hydrogen ions in the tomato juice is
times than the lemon juice.
a. 2; greater
b. 2; less
c. 100; less
d. 100; greater
Solution:
c
Free Response
Exercise:
Problem: Explain why water is an excellent solvent.
Solution:
Water molecules are polar, meaning they have separated partial
positive and negative charges. Because of these charges, water
molecules are able to surround charged particles created when a
substance dissociates. The surrounding layer of water molecules
stabilizes the ion and keeps differently charged ions from
reassociating, so the substance stays dissolved.
Glossary
acid
a substance that donates hydrogen ions and therefore lowers pH
adhesion
the attraction between water molecules and molecules of a different
substance
base
a substance that absorbs hydrogen ions and therefore raises pH
buffer
a solution that resists a change in pH by absorbing or releasing
hydrogen or hydroxide ions
cohesion
the intermolecular forces between water molecules caused by the polar
nature of water; creates surface tension
evaporation
the release of water molecules from liquid water to form water vapor
hydrophilic
describes a substance that dissolves in water; water-loving
hydrophobic
describes a substance that does not dissolve in water; water-fearing
litmus paper
filter paper that has been treated with a natural water-soluble dye so it
can be used as a pH indicator
PH scale
a scale ranging from 0 to 14 that measures the approximate
concentration of hydrogen ions of a substance
solvent
a substance capable of dissolving another substance
surface tension
the cohesive force at the surface of a body of liquid that prevents the
molecules from separating
temperature
a measure of molecular motion
Biological Macromolecules
By the end of this section, you will be able to:
e Describe the ways in which carbon is critical to life
e Explain the impact of slight changes in amino acids on organisms
¢ Describe the four major types of biological molecules
e Understand the functions of the four major types of molecules
The large molecules necessary for life that are built from smaller organic
molecules are called biological macromolecules. There are four major
classes of biological macromolecules (carbohydrates, lipids, proteins, and
nucleic acids), and each is an important component of the cell and performs
a wide array of functions. Combined, these molecules make up the majority
of a cell’s mass. Biological macromolecules are organic, meaning that they
contain carbon. In addition, they may contain hydrogen, oxygen, nitrogen,
phosphorus, sulfur, and additional minor elements.
Carbon
It is often said that life is “carbon-based.” This means that carbon atoms,
bonded to other carbon atoms or other elements, form the fundamental
components of many, if not most, of the molecules found uniquely in living
things. Other elements play important roles in biological molecules, but
carbon certainly qualifies as the “foundation” element for molecules in
living things. It is the bonding properties of carbon atoms that are
responsible for its important role.
Carbon Bonding
Carbon contains four electrons in its outer shell. Therefore, it can form four
covalent bonds with other atoms or molecules. The simplest organic carbon
molecule is methane (CH), in which four hydrogen atoms bind to a carbon
atom ([Link]).
Methane
Cy
On” moO
V4
Carbon can form four covalent bonds to
create an organic molecule. The simplest
carbon molecule is methane (CH,), depicted
here.
However, structures that are more complex are made using carbon. Any of
the hydrogen atoms can be replaced with another carbon atom covalently
bonded to the first carbon atom. In this way, long and branching chains of
carbon compounds can be made ((link]a). The carbon atoms may bond with
atoms of other elements, such as nitrogen, oxygen, and phosphorus
({link]b). The molecules may also form rings, which themselves can link
with other rings ([link]c). This diversity of molecular forms accounts for the
diversity of functions of the biological macromolecules and is based to a
large degree on the ability of carbon to form multiple bonds with itself and
other atoms.
Oo H H
YS lt Zs
C-C-N
74 | \
HO H H
(b)
i
H-C-OH
H ¢ e H
‘c H Bt 7
7 N
HO SY YH H ff OH
ya \
H OH
(c)
These examples show three
molecules (found in living
organisms) that contain carbon
atoms bonded in various ways
to other carbon atoms and the
atoms of other elements. (a)
This molecule of stearic acid
has a long chain of carbon
atoms. (b) Glycine, a
component of proteins, contains
carbon, nitrogen, oxygen, and
hydrogen atoms. (c) Glucose, a
sugar, has a ring of five carbon
atoms and one oxygen atom.
The chemical formula for
glucose is CgH)20¢
Carbohydrates
Carbohydrates are macromolecules with which most consumers are
somewhat familiar. To lose weight, some individuals adhere to “low-carb”
diets. Athletes, in contrast, often “carb-load” before important competitions
to ensure that they have sufficient energy to compete at a high level.
Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and
vegetables are all natural sources of carbohydrates. Carbohydrates provide
energy to the body, particularly through glucose, a simple sugar.
Carbohydrates also have other important functions in humans, animals, and
plants.
Carbohydrates can be represented by the formula (CH»O),, where n is the
number of carbon atoms in the molecule. In other words, the ratio of carbon
to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. Carbohydrates
are classified into three subtypes: monosaccharides, disaccharides, and
polysaccharides.
Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple sugars,
the most common of which is glucose. In monosaccharides, the number of
carbon atoms usually ranges from three to six. Most monosaccharide names
end with the suffix -ose. Depending on the number of carbon atoms in the
sugar, they may be known as trioses (three carbon atoms), pentoses (five
carbon atoms), and hexoses (six carbon atoms).
The chemical formula for glucose is CgH; Og. In most living species,
glucose is an important source of energy. During cellular respiration, energy
is released from glucose, and that energy is used to help make adenosine
triphosphate (ATP). Plants synthesize glucose using carbon dioxide and
water by the process of photosynthesis, and the glucose, in turn, is used for
the energy requirements of the plant. The excess synthesized glucose is
often stored as starch that is broken down by other organisms that feed on
plants.
Galactose (part of lactose, or milk sugar) and fructose (found in fruit) are
other common monosaccharides. Although glucose, galactose, and fructose
all have the same chemical formula (CgH, Og), they differ structurally and
chemically (and are known as isomers) because of differing arrangements
of atoms in the carbon chain ([link)).
Glucose Galactose Fructose
Glucose, galactose, and fructose are
isomeric monosaccharides, meaning that
they have the same chemical formula but
slightly different structures.
Disaccharides (di- = “two”) form when two monosaccharides undergo a
dehydration-synthesis reaction (a reaction in which the removal of a water
molecule occurs). During this process, the hydroxyl group (-OH) of one
monosaccharide combines with a hydrogen atom of another
monosaccharide, releasing a molecule of water (H»O) and forming a
covalent bond between atoms in the two sugar molecules.
Common disaccharides include lactose, maltose, and sucrose. Lactose is a
disaccharide consisting of the monomers glucose and galactose. It is found
naturally in milk. Maltose, or malt sugar, is a disaccharide formed from a
dehydration reaction between two glucose molecules. The most common
disaccharide is sucrose, or table sugar, which is composed of the monomers
glucose and fructose.
A long chain of monosaccharides linked by covalent bonds is known as a
polysaccharide (poly- = “many”). The chain may be branched or
unbranched, and it may contain different types of monosaccharides.
Polysaccharides may be very large molecules. Starch, glycogen, and
cellulose are examples of polysaccharides.
Starch is the stored form of sugars in plants and is made up of amylose and
amylopectin (both polymers of glucose). Plants are able to synthesize
glucose, and the excess glucose is stored as starch in different plant parts,
including roots and seeds. The starch that is consumed by animals is broken
down into smaller molecules, such as glucose. The cells can then absorb the
glucose.
Glycogen is the storage form of glucose in humans and other vertebrates,
and is made up of monomers of glucose. Glycogen is the animal equivalent
of starch and is a highly branched molecule usually stored in liver and
muscle cells as a form of stored energy. Whenever glucose levels decrease,
glycogen is broken down to release glucose. In the case of muscle cells, the
glucose is used to produce ATP for energy-requiring processes. In the case
of the liver, the glucose is released into the circulatory system to maintain
blood sugar homeostasis.
Cellulose is one of the most abundant natural biopolymers. The cell walls
of plants are mostly made of cellulose, which provides structural support to
the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made
up of glucose monomers that are linked by bonds between particular carbon
atoms in the glucose molecule.
Every other glucose monomer in cellulose is flipped over and packed
tightly as extended long chains. This gives cellulose its rigidity and high
tensile strength—which is so important to plant cells. Cellulose passing
through our digestive system is called dietary fiber. While the glucose-
glucose bonds in cellulose cannot be broken down by human digestive
enzymes, animals such as cows, buffalos, and horses (examples of
ruminants) are able to digest grass that is rich in cellulose and use it as a
food source. In these animals, certain species of bacteria reside in the rumen
(a part of their digestive system) and secrete the enzyme cellulase. The
appendix also contains bacteria that break down cellulose, giving it an
important role in the digestive systems of ruminants. Cellulases can break
down cellulose into glucose monomers that can be used as an energy source
by the animal.
Thus, through differences in molecular structure, carbohydrates are able to
serve the very different functions of energy storage (starch and glycogen)
and structural support and protection (cellulose) ({link]).
Starch Glycogen
CH 20H CH20H CH20OH CH20H CH20H
) Oo, O, ro)
OH OH OH OH Hd \OH °°
5 0 a 7 if CH,OH
OH OH OH OH OH O
OH ?
CH20H CH CH 20H CH20H
ie] ©. 0. Oo.
OH OH OH OH
HO O ie) ie} OH
OH OH OH
Cellulose Chitin
ie ny
oO fis X fis
‘ “
ra iia
ie Cho! eu
ir fe) og OH
CH,OH CH,OH
Although their structures and functions differ, all
polysaccharide carbohydrates are made up of
monosaccharides and have the chemical formula
(CH,O)n.
Note:
Careers in Action
Registered Dietitian
Obesity is a worldwide health concern, and many diseases, such as
diabetes and heart disease, are becoming more prevalent because of
obesity. This is one of the reasons why registered dietitians are increasingly
sought after for advice. Registered dietitians help plan food and nutrition
programs for individuals in various settings. They often work with patients
in health-care facilities, designing nutrition plans to prevent and treat
diseases. For example, dietitians may teach a patient with diabetes how to
manage blood-sugar levels by eating the correct types and amounts of
carbohydrates. Dietitians may also work in nursing homes, schools, and
private practices.
To become a registered dietitian, one needs to earn at least a bachelor’s
degree in dietetics, nutrition, food technology, or a related field. In
addition, registered dietitians must complete a supervised internship
program and pass a national exam. Those who pursue careers in dietetics
take courses in nutrition, chemistry, biochemistry, biology, microbiology,
and human physiology. Dietitians must become experts in the chemistry
and functions of food (proteins, carbohydrates, and fats).
Lipids
Lipids include a diverse group of compounds that are united by a common
feature. Lipids are hydrophobic (“water-fearing”), or insoluble in water,
because they are nonpolar molecules. This is because they are hydrocarbons
that include only nonpolar carbon-carbon or carbon-hydrogen bonds. Lipids
perform many different functions in a cell. Cells store energy for long-term
use in the form of lipids called fats (or triglycerides). Lipids also provide
insulation from the environment for plants and animals ([link]). For
example, they help keep aquatic birds and mammals dry because of their
water-repelling nature. Lipids are also the building blocks of many
hormones and are an important constituent of the plasma membrane. Lipids
include fats, oils, phospholipids, and steroids.
Hydrophobic lipids in the fur of
aquatic mammals, such as this river
otter, protect them from the elements.
(credit: Ken Bosma)
A fat molecule, such as a triglyceride, consists of two main components—
glycerol and fatty acids. Glycerol is an organic compound with three carbon
atoms, five hydrogen atoms, and three hydroxyl (-OH) groups. Fatty acids
have a long chain of hydrocarbons to which an acidic carboxyl group (-
COOH) is attached, hence the name “fatty acid.” The number of carbons in
the fatty acid may range from 4 to 36; most common are those containing
12-18 carbons. In a fat molecule, a fatty acid is attached to each of the three
oxygen atoms in the -OH groups of the glycerol molecule with a covalent
bond ([link])created by dehydration-synthesis reactions.
Saturated fatty acid Triglyceride
Ho Ho Ho Hp
H H H H H Cc Cc Cc Cc CH
HO RR Rt Ue Ser ae ae ee
c iG C Cc fe H—C—O~ He He He He
i Hp H> He Hp He Ho Ho Hp Ho
ie ie ie ie le
a
H—C—O Ho Hp H2 Hp
Ho H2 H2 H2
7 Bee on Be i er aa Oe ae a
nsaturated fatty aci H.C wv H—-¢—07 Ho Hp Hp Hp
H
CH>
Hoc
a a eo ae
No Ne No No NcoF ee Steroid
|| H2 Ho Ho H it
2 He He He
HoC Cc
‘ ne Hii \
CH2 H CH2
O- ipi Za C.
Phospholipid HoC b ZA we ZN Yom
O-—P—O-CH, Cc HC Hp
8 pee | I! LU
Hae wor ee
a a a a ae a
Heb Nae a Se Se ee
H2 Hp H2 He Hp
CN H> H Ho Hp Hp Hp Ho He
PO oh Oe gn PO OL OP On,
H,C—O Cc Cc Cc Cc fe C Cc fe CHs
He Ho Ho Ho He Hp Ho He
Lipids include fats, such as triglycerides, which are made
up of fatty acids and glycerol, phospholipids, and
steroids.
During this covalent bond formation, three water molecules are released.
The three fatty acids in the fat may be similar or dissimilar. These fats are
also called triglycerides because they have three fatty acids. Some fatty
acids have common names that specify their origin. For example, palmitic
acid, a saturated fatty acid, is derived from the palm tree. Arachidic acid is
derived from Arachis hypogaea, the scientific name for peanuts.
Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there
are only single bonds between neighboring carbons in the hydrocarbon
chain, the fatty acid is saturated. Saturated fatty acids are saturated with
hydrogen; in other words, the number of hydrogen atoms attached to the
carbon skeleton is maximized. When the hydrocarbon chain contains a
double bond, the fatty acid is an unsaturated fatty acid.
Most unsaturated fats are liquid at room temperature and are called oils. If
there is only one carbon-carbon double bond in the molecule, then it is
known as a monounsaturated fat (e.g. olive oil), and if there is more than
one carbon-carbon double bond, then it is known as a polyunsaturated fat
(e.g. canola oil).
Saturated fats tend to get packed tightly and are solid at room temperature.
Animal fats with stearic acid and palmitic acid contained in meat, and the
fat with butyric acid contained in butter, are examples of saturated fats.
Mammals store fats in specialized cells called adipocytes, where globules of
fat occupy most of the cell. In plants, fat or oil is stored in seeds and is used
as a source of energy during embryonic development.
Unsaturated fats or oils are usually of plant origin and contain unsaturated
fatty acids. The double bond causes a bend or a “kink” that prevents the
fatty acids from packing tightly, keeping them liquid at room temperature.
Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated
fats. Unsaturated fats help to improve blood cholesterol levels, whereas
saturated fats contribute to plaque formation in the arteries, which increases
the risk of a heart attack.
In the food industry, oils are artificially hydrogenated to make them semi-
solid, leading to less spoilage and increased shelf life. Simply speaking,
hydrogen gas is bubbled through oils to solidify them. During this
hydrogenation process, double bonds of the cis-conformation in the
hydrocarbon chain may be converted to double bonds in the trans-
conformation. This forms a trans-fat from a cis-fat. The orientation of the
double bonds affects the chemical properties of the fat ([link]).
trans-fat molecule
During the hydrogenation process,
the orientation around the double
bonds is changed, making a trans-
fat from a cis-fat. This changes the
chemical properties of the
molecule.
Margarine, some types of peanut butter, and shortening are examples of
artificially hydrogenated trans-fats. Recent studies have shown that an
increase in trans-fats in the human diet may lead to an increase in levels of
low-density lipoprotein (LDL), or “bad” cholesterol, which, in turn, may
lead to plaque deposition in the arteries, resulting in heart disease. Many
fast food restaurants have recently eliminated the use of trans-fats, and U.S.
food labels are now required to list their trans-fat content.
Salmon, trout, and tuna are good sources of omega-3 fatty acids. Omega-3
fatty acids are important in brain function and normal growth and
development. They may also prevent heart disease and reduce the risk of
cancer.
Like carbohydrates, fats have received a lot of bad publicity. It is true that
eating an excess of fried foods and other “fatty” foods leads to weight gain.
However, fats do have important functions. Fats serve as long-term energy
storage. They also provide insulation for the body. Therefore, “healthy”
unsaturated fats in moderate amounts should be consumed on a regular
basis.
Phospholipids are the major constituent of the plasma membrane. Like
fats, they are composed of fatty acid chains attached to a glycerol or similar
backbone. Instead of three fatty acids attached, however, there are two fatty
acids and the third carbon of the glycerol backbone is bound to a phosphate
group. The phosphate group is modified by the addition of an alcohol.
A phospholipid has both hydrophobic and hydrophilic regions. The fatty
acid chains are hydrophobic and exclude themselves from water, whereas
the phosphate is hydrophilic and interacts with water.
Cells are surrounded by a membrane, which has a bilayer of phospholipids.
The fatty acids of phospholipids face inside, away from water, whereas the
phosphate group can face either the outside environment or the inside of the
cell, which are both aqueous.
Steroids
Unlike the phospholipids and fats discussed earlier, steroids have a ring
structure. Although they do not resemble other lipids, they are grouped with
them because they are also hydrophobic. All steroids have four, linked
carbon rings and several of them, like cholesterol, have a short tail.
Cholesterol is a steroid. Cholesterol is mainly synthesized in the liver and is
the precursor of many steroid hormones, such as testosterone and estradiol.
It is also the precursor of vitamins E and K. Cholesterol is the precursor of
bile salts, which help in the breakdown of fats and their subsequent
absorption by cells. Although cholesterol is often spoken of in negative
terms, it is necessary for the proper functioning of the body. It is a key
component of the plasma membranes of animal cells.
Note:
Concept in Action
[ml
a
rat le)
i.
openstax COLLEGE
.
For an additional perspective on lipids, explore “Biomolecules: The
Lipids” through this interactive animation.
Proteins
Proteins are one of the most abundant organic molecules in living systems
and have the most diverse range of functions of all macromolecules.
Proteins may be structural, regulatory, contractile, or protective; they may
serve in transport, storage, or membranes; or they may be toxins or
enzymes. Each cell in a living system may contain thousands of different
proteins, each with a unique function. Their structures, like their functions,
vary greatly. They are all, however, polymers of amino acids, arranged in a
linear sequence.
The functions of proteins are very diverse because there are 20 different
chemically distinct amino acids that form long chains, and the amino acids
can be in any order. For example, proteins can function as enzymes or
hormones. Enzymes, which are produced by living cells, are catalysts in
biochemical reactions (like digestion) and are usually proteins. Catalysts
speed up chemical reactions by lowering the amount of energy required to
Start them. Each enzyme is specific for the substrate (a reactant that binds to
an enzyme) upon which it acts. Enzymes can function to break molecular
bonds, to rearrange bonds, or to form new bonds. An example of an enzyme
is salivary amylase, which breaks down amylose, a component of starch.
Hormones are chemical signaling molecules, usually proteins or steroids,
secreted by an endocrine gland or group of endocrine cells that act to
control or regulate specific physiological processes, including growth,
development, metabolism, and reproduction. For example, insulin is a
protein hormone that maintains blood glucose levels.
Proteins have different shapes and molecular weights; some proteins are
globular in shape whereas others are fibrous in nature. For example,
hemoglobin is a globular protein, but collagen, found in our skin, is a
fibrous protein. Protein shape is critical to its function. Changes in
temperature, pH, and exposure to chemicals may lead to permanent changes
in the shape of the protein, leading to a loss of function or denaturation (to
be discussed in more detail later). All proteins are made up of different
arrangements of the same 20 kinds of amino acids.
Amino acids are the monomers that make up proteins. Each amino acid has
the same fundamental structure, which consists of a central carbon atom
bonded to an amino group (—NH>), a carboxyl group (-COOH), and a
hydrogen atom. Every amino acid also has another variable atom or group
of atoms bonded to the central carbon atom known as the R group. The R
group is the only difference in structure between the 20 amino acids;
otherwise, the amino acids are identical ({link]).
Fundamental structure
Hydrogen
; H
Amino | Carboxyl
group group
H2N——C——COOH
Alanine Valine
H H
H2N—— C——COOH H2N—— C——COOH
CH3
Lysine Aspartic acid
H
H2N——C—COOH H»aN—C—COOH
(CH2)4 CH2
NH2
Amino acids are made up of a
central carbon bonded to an
amino group (—NH>), a
carboxyl group (-COOH), and a
hydrogen atom. The central
carbon’s fourth bond varies
among the different amino
acids, as seen in these examples
of alanine, valine, lysine, and
aspartic acid.
The chemical nature of the R group determines the chemical nature of the
amino acid within its protein (that is, whether it is acidic, basic, polar, or
nonpolar).
The sequence and number of amino acids ultimately determine a protein’s
shape, size, and function. Each amino acid is attached to another amino acid
by a covalent bond, known as a peptide bond, which is formed by a
dehydration reaction. The carboxyl group of one amino acid and the amino
group of a second amino acid combine, releasing a water molecule. The
resulting bond is the peptide bond.
The products formed by such a linkage are called polypeptides. While the
terms polypeptide and protein are sometimes used interchangeably, a
polypeptide is technically a polymer of amino acids, whereas the term
protein is used for a polypeptide or polypeptides that have combined
together, have a distinct shape, and have a unique function.
Protein Structure
As discussed earlier, the shape of a protein is critical to its function. To
understand how the protein gets its final shape or conformation, we need to
understand the four levels of protein structure: primary, secondary, tertiary,
and quaternary ([link]).
The unique sequence and number of amino acids in a polypeptide chain is
its primary structure. The unique sequence for every protein is ultimately
determined by the gene (i.e. a section of DNA) that encodes (i.e. has the
information to make) the protein. Any change in the gene sequence may
lead to a different amino acid being added to the polypeptide chain, causing
a change in protein structure and function. In sickle cell anemia, the
hemoglobin B chain has a single amino acid substitution, causing a change
in both the structure and function of the protein. What is most remarkable to
consider is that a hemoglobin molecule is made up of two alpha chains and
two beta chains that each consist of about 150 amino acids. The molecule,
therefore, has about 600 amino acids. The structural difference between a
normal hemoglobin molecule and a sickle cell molecule—that dramatically
decreases life expectancy in the affected individuals—is a single amino acid
of the 600.
Because of this change of one amino acid in the chain, the normally
biconcave, or disc-shaped, red blood cells assume a crescent or “sickle”
shape, which clogs arteries. This can lead to a myriad of serious health
problems, such as breathlessness, dizziness, headaches, and abdominal pain
for those who have this disease.
Folding patterns resulting from interactions between the non-R group
portions of amino acids give rise to the secondary structure of the protein.
The most common are the alpha (a)-helix and beta (f)-pleated sheet
structures. Both structures are held in shape by hydrogen bonds. In the
alpha helix, the bonds form between every fourth amino acid and cause a
twist in the amino acid chain.
In the B-pleated sheet, the “pleats” are formed by hydrogen bonding
between atoms on the backbone of the polypeptide chain. The R groups are
attached to the carbons, and extend above and below the folds of the pleat.
The pleated segments align parallel to each other, and hydrogen bonds form
between the same pairs of atoms on each of the aligned amino acids. The a-
helix and B-pleated sheet structures are found in many globular and fibrous
proteins.
The unique three-dimensional structure of a polypeptide is known as its
tertiary structure. This structure is caused by chemical interactions between
various amino acids and regions of the polypeptide. Primarily, the
interactions among R groups create the complex three-dimensional tertiary
structure of a protein. There may be ionic bonds formed between R groups
on different amino acids, or hydrogen bonding beyond that involved in the
secondary structure. When protein folding takes place, the hydrophobic R
groups of nonpolar amino acids lay in the interior of the protein, whereas
the hydrophilic R groups lay on the outside. The former types of
interactions are also known as hydrophobic interactions.
In nature, some proteins are formed from several polypeptides, also known
as subunits, and the interaction of these subunits forms the quaternary
structure. Weak interactions between the subunits help to stabilize the
overall structure. For example, hemoglobin is a combination of four
polypeptide subunits.
Amino acids
Primary protein structure
sequence of a chain of
amino acids
Beta-pleated Secondary protein structure
sheet hydrogen bonding of the peptide
backbone causes the amino
acids to fold into a repeating
pattern
Beta-pleated
sheet
Alpha helix
The four levels of protein structure can
be observed in these illustrations. (credit:
modification of work by National Human
Genome Research Institute)
Quaternary protein structure
protein consisting of more
than one amino acid chain
Each protein has its own unique sequence and shape held together by
chemical interactions. If the protein is subject to changes in temperature,
pH, or exposure to chemicals, the protein structure may change, losing its
shape in what is known as denaturation. Denaturation is often reversible
because the primary structure is preserved if the denaturing agent is
removed, allowing the protein to resume its function. Sometimes
denaturation is irreversible, leading to a loss of function. One example of
protein denaturation can be seen when an egg is fried or boiled. The
albumin protein in the liquid egg white is denatured when placed in a hot
pan, changing from a clear substance to an opaque white substance. Not all
proteins are denatured at high temperatures; for instance, bacteria that
survive in hot springs have proteins that are adapted to function at those
temperatures.
Note:
Concept in Action
ti
= openstax COLLEGE
even
For an additional perspective on proteins, explore “Biomolecules: The
Proteins” through this interactive animation.
Nucleic Acids
Nucleic acids are key macromolecules in the continuity of life. They carry
the genetic blueprint of a cell and carry instructions for the functioning of
the cell.
The two main types of nucleic acids are deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA). DNA is the genetic material found in all living
organisms, ranging from single-celled bacteria to multicellular mammals.
The other type of nucleic acid, RNA, is mostly involved in protein
synthesis. The DNA molecules never leave the nucleus, but instead use an
RNA intermediary to communicate with the rest of the cell. Other types of
RNA are also involved in protein synthesis and its regulation.
DNA and RNA are made up of monomers known as nucleotides. The
nucleotides combine with each other to form a polynucleotide, DNA or
RNA. Each nucleotide is made up of three components: a nitrogenous base,
a pentose (five-carbon) sugar, and a phosphate group ((link]). Each
nitrogenous base in a nucleotide is attached to a sugar molecule, which is
attached to a phosphate group. DNA nucleotides contain the sugar
deoxyribose and one of the four bases adenine (A),thymine (T), guanine (G),
or cytosine (C). RNA nucleotides contain the sugar ribose and one of the
four bases A,uracil (U),G,or C.
N NH,
il Nitrogenous base
c= oO —_— a fe)
oO
Phosphate
OH
Sugar
A nucleotide is made up of three
components: a nitrogenous base, a
pentose sugar, and a phosphate group.
DNA Double-Helical Structure
DNA has a double-helical structure ({link]). It is composed of two strands,
or polymers, of nucleotides. Each strand is formed with bonds between
phosphate and sugar groups of adjacent nucleotides (called a
phosphodiester bond). The two strands are bonded to each other at their
bases with hydrogen bonds, and the strands coil about each other along their
length, hence the “double helix” description, which means a double spiral.
The double-helix
model shows DNA
as two parallel
strands of
intertwining
molecules. (credit:
Jerome Walker,
Dennis Myts)
The alternating sugar and phosphate groups lie on the outside of each
strand, forming the backbone of the DNA. The nitrogenous bases are
stacked in the interior, like the steps of a staircase, and these bases pair; the
pairs are bound to each other by hydrogen bonds. The bases pair in such a
way that the distance between the backbones of the two strands is the same
all along the molecule.
Section Summary
Living things are carbon-based because carbon plays such a prominent role
in the chemistry of living things. The four covalent bonding positions of the
carbon atom can give rise to a wide diversity of compounds with many
functions, accounting for the importance of carbon in living things.
Carbohydrates are a group of macromolecules that are a vital energy source
for the cell, provide structural support to many organisms, and can be found
on the surface of the cell as receptors or for cell recognition. Carbohydrates
are Classified as monosaccharides, disaccharides, and polysaccharides,
depending on the number of monomers in the molecule.
Lipids are a class of macromolecules that are nonpolar and hydrophobic in
nature. Major types include fats and oils, waxes, phospholipids, and
steroids. Fats and oils are a stored form of energy and can include
triglycerides. Fats and oils are usually made up of fatty acids and glycerol.
Proteins are a class of macromolecules that can perform a diverse range of
functions for the cell. They help in metabolism by providing structural
support and by acting as enzymes, carriers or as hormones. The building
blocks of proteins are amino acids. Proteins are organized at four levels:
primary, secondary, tertiary, and quaternary. Protein shape and function are
intricately linked; any change in shape caused by changes in temperature,
pH, or chemical exposure may lead to protein denaturation and a loss of
function.
Nucleic acids are molecules made up of repeating units of nucleotides that
direct cellular activities such as cell division and protein synthesis. Each
nucleotide is made up of a pentose sugar, a nitrogenous base, and a
phosphate group. There are two types of nucleic acids: DNA and RNA.
Multiple Choice
Exercise:
Problem: An example of a monosaccharide is
a. fructose
b. glucose
c. galactose
d. all of the above
Solution:
D
Exercise:
Problem:Cellulose and starch are examples of
a. monosaccharides
b. disaccharides
c. lipids
d. polysaccharides
Solution:
D
Exercise:
Problem: Phospholipids are important components of
a. the plasma membrane of cells
b. the ring structure of steroids
c. the waxy covering on leaves
d. the double bond in hydrocarbon chains
Solution:
A
Exercise:
Problem:The monomers that make up proteins are called
a. nucleotides
b. disaccharides
c. amino acids
d. chaperones
Solution:
C
Free Response
Exercise:
Problem:
Explain at least three functions that lipids serve in plants and/or
animals.
Solution:
Fat serves as a valuable way for animals to store energy. It can also
provide insulation. Phospholipids and steroids are important
components of cell membranes.
Exercise:
Problem:
Explain what happens if even one amino acid is substituted for another
in a polypeptide chain. Provide a specific example.
Solution:
A change in gene sequence can lead to a different amino acid being
added to a polypeptide chain instead of the normal one. This causes a
change in protein structure and function. For example, in sickle cell
anemia, the hemoglobin £6 chain has a single amino acid substitution.
Because of this change, the disc-shaped red blood cells assume a
crescent shape, which can result in serious health problems.
Glossary
amino acid
a monomer of a protein
carbohydrate
a biological macromolecule in which the ratio of carbon to hydrogen to
oxygen is 1:2:1; carbohydrates serve as energy sources and structural
support in cells
cellulose
a polysaccharide that makes up the cell walls of plants and provides
structural support to the cell
chitin
a type of carbohydrate that forms the outer skeleton of arthropods,
such as insects and crustaceans, and the cell walls of fungi
denaturation
the loss of shape in a protein as a result of changes in temperature, pH,
or exposure to chemicals
deoxyribonucleic acid (DNA)
a double-stranded polymer of nucleotides that carries the hereditary
information of the cell
disaccharide
two sugar monomers that are linked together by a peptide bond
enzyme
a catalyst in a biochemical reaction that is usually a complex or
conjugated protein
fat
a lipid molecule composed of three fatty acids and a glycerol
(triglyceride) that typically exists in a solid form at room temperature
glycogen
a storage carbohydrate in animals
hormone
a chemical signaling molecule, usually a protein or steroid, secreted by
an endocrine gland or group of endocrine cells; acts to control or
regulate specific physiological processes
lipids
a class of macromolecules that are nonpolar and insoluble in water
macromolecule
a large molecule, often formed by polymerization of smaller
monomers
monosaccharide
a single unit or monomer of carbohydrates
nucleic acid
a biological macromolecule that carries the genetic information of a
cell and carries instructions for the functioning of the cell
nucleotide
a monomer of nucleic acids; contains a pentose sugar, a phosphate
group, and a nitrogenous base
oil
an unsaturated fat that is a liquid at room temperature
phospholipid
a major constituent of the membranes of cells; composed of two fatty
acids and a phosphate group attached to the glycerol backbone
polypeptide
a long chain of amino acids linked by peptide bonds
polysaccharide
a long chain of monosaccharides; may be branched or unbranched
protein
a biological macromolecule composed of one or more chains of amino
acids
ribonucleic acid (RNA)
a single-stranded polymer of nucleotides that is involved in protein
synthesis
saturated fatty acid
a long-chain hydrocarbon with single covalent bonds in the carbon
chain; the number of hydrogen atoms attached to the carbon skeleton is
maximized
starch
a storage carbohydrate in plants
steroid
a type of lipid composed of four fused hydrocarbon rings
trans-fat
a form of unsaturated fat with the hydrogen atoms neighboring the
double bond across from each other rather than on the same side of the
double bond
triglyceride
a fat molecule; consists of three fatty acids linked to a glycerol
molecule
unsaturated fatty acid
a long-chain hydrocarbon that has one or more than one double bonds
in the hydrocarbon chain
Introduction
class="introduction'
(a) Nasal
sinus cells
(viewed with
a light
microscope),
(b) onion
cells (viewed
with a light
microscope),
and (c) Vibrio
tasmaniensis
bacterial cells
(viewed using
a scanning
electron
microscope)
are from very
different
organisms,
yet all share
certain
characteristic
s of basic cell
structure.
(credit a:
modification
of work by
Ed Uthman,
MD; credit b:
modification
of work by
Umberto
Salvagnin;
credit c:
modification
of work by
Anthony
D'Onofrio;
scale-bar data
from Matt
Russell)
Close your eyes and picture a brick wall. What is the basic building block
of that wall? It is a single brick, of course. Like a brick wall, your body is
composed of basic building blocks, and the building blocks of your body
are cells.
Your body has many kinds of cells, each specialized for a specific purpose.
Just as a home is made from a variety of building materials, the human
body is constructed from many cell types. For example, epithelial cells
protect the surface of the body and cover the organs and body cavities
within. Bone cells help to support and protect the body. Cells of the immune
system fight invading bacteria. Additionally, red blood cells carry oxygen
throughout the body. Each of these cell types plays a vital role during the
growth, development, and day-to-day maintenance of the body. In spite of
their enormous variety, however, all cells share certain fundamental
characteristics. In this chapter, these characteristics will be examined in
greater detail.
Prokaryotic and Eukaryotic Cells
By the end of this section, you will be able to:
e Name examples of prokaryotic and eukaryotic organisms
e Compare and contrast prokaryotic cells and eukaryotic cells
¢ Describe the relative sizes of different kinds of cells
Cells fall into one of two broad categories: prokaryotic and eukaryotic. The
predominantly single-celled organisms of the domains Bacteria and
Archaea are classified as prokaryotes (pro- = before; -karyon- = nucleus).
Animal cells, plant cells, fungi, and protists are eukaryotes (eu- = true).
Components of Prokaryotic Cells
All cells share four common components: 1) a plasma membrane, an outer
covering that separates the cell’s interior from its surrounding environment;
2) cytoplasm, consisting of a jelly-like region within the cell in which other
cellular components are found; 3) DNA, the genetic material of the cell; and
4) ribosomes, particles that synthesize proteins. However, prokaryotes differ
from eukaryotic cells in several ways.
A prokaryotic cell is a simple, single-celled (unicellular) organism that
lacks a nucleus, or any other membrane-bound organelle. We will shortly
come to see that this is significantly different in eukaryotes. Prokaryotic
DNA is found in the central part of the cell: a darkened region called the
nucleoid ([link]).
Cell
membrane
Chromosome Nucleoid region
(DNA)
This figure shows the generalized
structure of a prokaryotic cell.
Eukaryotic Cells
In nature, the relationship between form and function is apparent at all
levels, including the level of the cell, and this will become clear as we
explore eukaryotic cells. The principle “form follows function” is found in
many contexts. For example, birds and fish have streamlined bodies that
allow them to move quickly through the medium in which they live, be it
air or water. It means that, in general, one can deduce the function of a
structure by looking at its form, because the two are matched.
A eukaryotic cell is a cell that has a membrane-bound nucleus and other
membrane-bound compartments or sacs, called organelles, which have
specialized functions. The word eukaryotic means “true kernel” or “true
nucleus,” alluding to the presence of the membrane-bound nucleus in these
cells. The word “organelle” means “little organ,” and, as already mentioned,
organelles have specialized cellular functions, just as the organs of your
body have specialized functions.
Cell Size
At 0.1-5.0 micrometers (im; 1/1,000,000 of a meter) in diameter,
prokaryotic cells are significantly smaller than eukaryotic cells, which have
diameters ranging from 10-100 pm ({link]). The small size of prokaryotes
allows ions and organic molecules that enter them to quickly spread to other
parts of the cell. Similarly, any wastes produced within a prokaryotic cell
can quickly move out. However, larger eukaryotic cells have evolved
different structural adaptations to enhance cellular transport. Indeed, the
large size of these cells would not be possible without these adaptations. In
general, cell size is limited because volume increases much more quickly
than does cell surface area. This is because volume is a cubic dimension
and surface area is a squared dimension. For example, if X=2, then the
surface area (x squared) is 4 and the volume (x cubed) is 8. If x =3, then the
surface area is 9 and the volume is 27. As a cell becomes larger, it becomes
more and more difficult for the cell to acquire sufficient materials to support
the processes inside the cell, because the relative size of the surface area
across which materials must be transported declines.
Animal
cell
Mitochondria
Protein
iy
Lipids Ostrich ‘Adult
Bacteria egg female
0.1 nm inm 10 nm 100 nm 1yum 10 um 100 pm 1mm 10 mm 100 mm 1m
Naked eye
Light microscope
Electron microscope
This figure shows the relative sizes of different kinds of cells
and cellular components. An adult human is shown for
comparison. Note that a light microscope is required to view
both prokaryotic and eukaryotic cells. Note: 1000 nanometers
equals 1 micrometer, 1000 micrometers equals 1 millimeter,
and 1000 millimeters equals one meter.
Section Summary
Prokaryotes are predominantly single-celled organisms of the domains
Bacteria and Archaea. All prokaryotes have plasma membranes, cytoplasm,
ribosomes, a cell wall, DNA, and lack membrane-bound organelles. Many
also have polysaccharide capsules. Prokaryotic cells range in diameter from
0.1—5.0 ppm.
Like a prokaryotic cell, a eukaryotic cell has a plasma membrane,
cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a
prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a
membrane), and has other membrane-bound organelles that allow for
compartmentalization of functions. Eukaryotic cells tend to be 10 to 100
times the size of prokaryotic cells.
Multiple Choice
Exercise:
Problem: Which of these do all prokaryotes and eukaryotes share?
a. nuclear envelope
b. cell walls
c. organelles
d. plasma membrane
Solution:
D
Exercise:
Problem:
A typical prokaryotic cell compared to a
eukaryotic cell.
a. is smaller in size by a factor of 100
b. is similar in size
c. is smaller in size by a factor of one million
d. is larger in size by a factor of 10
Solution:
A
Free Response
Exercise:
Problem:
What are organelles and which type of cells (prokaryotic or
eukaryotic) contains them?
Solution:
Organelles are membrane-bound compartments or sacs that have
specialized functions in eukaryotic cells.
Glossary
eukaryotic cell
a cell that has a membrane-bound nucleus and several other
membrane-bound compartments or sacs
organelle
a membrane-bound compartment or sac within a cell
prokaryotic cell
a unicellular organism that lacks a nucleus or any other membrane-
bound organelle
A More Detailed Look at Eukaryotic Cells
By the end of this section, you will be able to:
e Describe the structure of eukaryotic animal cells
e State the role of the plasma membrane
e Summarize the functions of the major cell organelles
e Describe the cytoskeleton and extracellular matrix
At this point, it should be clear that eukaryotic cells have a more complex
structure than do prokaryotic cells. Organelles and other cellular
components allow for various functions to occur in the cell at the same
time. Before discussing the functions of organelles within a eukaryotic cell,
let us first examine two important components of the cell: the plasma
membrane and the cytoplasm.
Note:
Art Connection
This figure shows a typical animal cell.
Nucleus Cytoskeleton
Nuclear envelope: Microtubules: form the
membrane enclosing | mitotic spindle and
the nucleus. Protein-lined maintain cell shape.
pores allow material to Centrosome: microtubule-
move in and out. organizing center.
Chromatin: DNA plus Intermediate filaments:
associated proteins. fibrous proteins that hold
Nucleolus: organelles in place.
condensed region Microfilaments:
where ribosomes fibrous proteins;
are formed. form the cellular
cortex.
Peroxisome: Plasma
metabolizes membrane
waste
Lysosome:
digests food and
waste materials.
Golgi apparatus:
modifies proteins.
Endoplasmic
reticulum
Rough: associated
with ribosomes;
makes secretory and
membrane proteins.
Smooth: makes lipids.
Cytoplasm
Mitochondria:
produce energy.
(a)
The Plasma Membrane
Like prokaryotes, eukaryotic cells have a plasma membrane ((link]) made
up of a phospholipid bilayer with embedded proteins that separates the
internal contents of the cell from its surrounding environment. A
phospholipid is a lipid molecule composed of two fatty acid chains, a
glycerol backbone, and a phosphate group. The plasma membrane regulates
the passage of some substances, such as organic molecules, ions, and water,
preventing the passage of some to maintain internal conditions, while
actively bringing in or removing others. Other compounds move passively
across the membrane.
Glycoprotein: protein with Glycolipid: lipid with
lak carbohydrate attached J carbohydrate
attached
Peripheral membrane Phospholipid
protein bilayer
Integral membrane Cholesterol ,
protein i Protein channel
Filaments of the cytoskeleton
The plasma membrane is a phospholipid bilayer with
embedded proteins. There are other components, such as
cholesterol and carbohydrates, which can be found in the
membrane in addition to phospholipids and protein. The phrase
"fluid mosaic" is used to describe the structure of the plasma
membrane because it is dynamic and contains numerous
components.
The plasma membranes of cells that specialize in absorption are folded into
fingerlike projections called microvilli (singular = microvillus). This
folding increases the surface area of the plasma membrane. Such cells are
typically found lining the small intestine, the organ that absorbs nutrients
from digested food. This is an excellent example of form matching the
function of a structure.
People with celiac disease have an immune response to gluten, which is a
protein found in wheat, barley, and rye. The immune response damages
microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads
to malnutrition, cramping, and diarrhea. Patients suffering from celiac
disease must follow a gluten-free diet.
The Cytoplasm
The cytoplasm comprises the contents of a cell between the plasma
membrane and the nuclear envelope (a structure to be discussed shortly). It
is made up of organelles suspended in the gel-like cytosol, the cytoskeleton,
and various chemicals. Even though the cytoplasm consists of 70 to 80
percent water, it has a semi-solid consistency, which comes from the
proteins within it. However, proteins are not the only organic molecules
found in the cytoplasm. Glucose and other simple sugars, polysaccharides,
amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found
there too. Ions of sodium, potassium, calcium, and many other elements are
also dissolved in the cytoplasm. Many metabolic reactions, including
protein synthesis, take place in the cytoplasm.
The Endomembrane System
The endomembrane system (endo = within) is a group of membranes and
organelles ({link]) in eukaryotic cells that work together to modify, package,
and transport lipids and proteins. It includes the nuclear envelope,
lysosomes, and vesicles, the endoplasmic reticulum and Golgi apparatus,
which we will cover shortly. Although not technically within the cell, the
plasma membrane is included in the endomembrane system because, as you
will see, it interacts with the other endomembranous organelles.
The Nucleus
Typically, the nucleus is the most prominent organelle in a cell. The
nucleus (plural = nuclei) houses the cell’s DNA in the form of chromatin
and directs the synthesis of ribosomes and proteins. Let us look at it in more
detail ({link]).
Endoplasmic
reticulum
Nucleolus
Chromatin
Nucleoplasm
Nuclear pore
Nuclear envelope
The outermost boundary of the
nucleus is the nuclear envelope.
Notice that the nuclear envelope
consists of two phospholipid bilayers
(membranes)—an outer membrane
and an inner membrane—in contrast
to the plasma membrane ((link]),
which consists of only one
phospholipid bilayer. (credit:
modification of work by NIGMS,
NIH)
The nuclear envelope is a double-membrane structure that constitutes the
outermost portion of the nucleus ([link]). Both the inner and outer
membranes of the nuclear envelope are phospholipid bilayers.
The nuclear envelope is punctuated with pores that control the passage of
ions, molecules, and RNA between the nucleoplasm and the cytoplasm. The
DNA in the nucleus is too large to fit through the pores.
To understand chromatin, it is helpful to first consider chromosomes.
Chromosomes are structures within the nucleus that are made up of DNA
(the hereditary material) and proteins. This combination of DNA and
proteins is called chromatin. In eukaryotes, chromosomes are linear
structures. Every species has a specific number of chromosomes in the
nucleus of its body cells. For example, in humans, the chromosome number
is 46, whereas in fruit flies, the chromosome number is eight.
Chromosomes are only visible and distinguishable from one another when
the cell is getting ready to divide. This is because the DNA condenses or
compacts in preparation for cell division. When the cell is in the growth and
maintenance phases of its life cycle, the chromosomes resemble an
unwound, jumbled bunch of threads and the DNA is more accessible to be
used to make proteins.
We already know that the nucleus directs the synthesis of ribosomes, but
how does it do this? Some chromosomes have sections of DNA that encode
ribosomal RNA. A darkly staining area within the nucleus, called the
nucleolus (plural = nucleoli), aggregates the ribosomal RNA with
associated proteins to assemble the ribosomal subunits that are then
transported through the nuclear pores into the cytoplasm.
The Endoplasmic Reticulum
The endoplasmic reticulum (ER) ((link]) is a series of interconnected
membranous tubules that collectively modify proteins and synthesize lipids.
However, these two functions are performed in separate areas of the
endoplasmic reticulum: the rough endoplasmic reticulum and the smooth
endoplasmic reticulum, respectively.
The rough endoplasmic reticulum (RER) is so named because the
ribosomes attached to its cytoplasmic surface give it a studded appearance
when viewed through an electron microscope.
The ribosomes synthesize proteins while attached to the ER, resulting in
transfer of their newly synthesized proteins into the lumen of the RER
where they undergo modifications such as folding or addition of sugars. The
RER also makes phospholipids for cell membranes.
If the phospholipids or modified proteins are not destined to stay in the
RER, they will be packaged within vesicles and transported from the RER
by budding from the membrane ([link]). Since the RER is engaged in
modifying proteins that will be secreted from the cell, it is abundant in cells
that secrete proteins, such as the liver.
The smooth endoplasmic reticulum (SER) is continuous with the RER
but has few or no ribosomes on its cytoplasmic surface (see [link]). The
SER’s functions include synthesis of carbohydrates, lipids (including
phospholipids), and steroid hormones; detoxification of medications and
poisons; alcohol metabolism; and storage of calcium ions.
The Golgi Apparatus
We have already mentioned that vesicles can bud from the ER, but where
do the vesicles go? Before reaching their final destination, the lipids or
proteins within the transport vesicles need to be sorted, packaged, and
tagged so that they wind up in the right place. The sorting, tagging,
packaging, and distribution of lipids and proteins take place in the Golgi
apparatus (also called the Golgi body), a series of flattened membranous
sacs ([link]).
18 e
~s 8 trans face °_*
7 iad
*
Golgi apparatus
&
The Golgi apparatus in this transmission
electron micrograph of a white blood cell is
visible as a stack of semicircular flattened
rings in the lower portion of this image.
Several vesicles can be seen near the Golgi
apparatus. (credit: modification of work by
Louisa Howard; scale-bar data from Matt
Russell)
The Golgi apparatus has a receiving face near the endoplasmic reticulum
and a releasing face on the side away from the ER, toward the cell
membrane. The transport vesicles that form from the ER travel to the
receiving face, fuse with it, and empty their contents into the lumen of the
Golgi apparatus. As the proteins and lipids travel through the Golgi, they
undergo further modifications. The most frequent modification is the
addition of short chains of sugar molecules. The newly modified proteins
and lipids are then tagged with small molecular groups to enable them to be
routed to their proper destinations.
Finally, the modified and tagged proteins are packaged into vesicles that
bud from the opposite face of the Golgi. While some of these vesicles,
transport vesicles, deposit their contents into other parts of the cell where
they will be used, others, secretory vesicles, fuse with the plasma
membrane and release their contents outside the cell.
The amount of Golgi in different cell types again illustrates that form
follows function within cells. Cells that engage in a great deal of secretory
activity (such as cells of the salivary glands that secrete digestive enzymes
or cells of the immune system that secrete antibodies) have an abundant
number of Golgi.
Lysosomes
cc
In animal cells, the lysosomes are the cell’s “garbage disposal.” Digestive
enzymes within the lysosomes aid the breakdown of proteins,
polysaccharides, lipids, nucleic acids, and even worn-out organelles. In
single-celled eukaryotes, lysosomes are important for digestion of the food
they ingest and the recycling of organelles. These enzymes are active at a
much lower pH (more acidic) than those located in the cytoplasm. Many
reactions that take place in the cytoplasm could not occur at a low pH, thus
the advantage of compartmentalizing the eukaryotic cell into organelles is
apparent.
Lysosomes also use their hydrolytic enzymes to destroy disease-causing
organisms that might enter the cell. A good example of this occurs in a
group of white blood cells called macrophages, which are part of your
body’s immune system. In a process known as phagocytosis, a section of
the plasma membrane of the macrophage invaginates (folds in) and engulfs
a pathogen. The invaginated section, with the pathogen inside, then pinches
itself off from the plasma membrane and becomes a vesicle. The vesicle
fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the
pathogen ({link]).
Bacterium Vesicle
_\
Macrophage
A macrophage has phagocytized a
potentially pathogenic bacterium into
a vesicle, which then fuses with a
lysosome within the cell so that the
pathogen can be destroyed. Other
organelles are present in the cell, but
for simplicity, are not shown.
Vesicles
Vesicles are membrane-bound sacs that function in storage and transport.
Vesicles can fuse with other membranes within the cell system.
Note:
Art Connection
Plasma membrane
The endomembrane system works to modify,
package, and transport lipids and proteins.
(credit: modification of work by Magnus
Manske)
Ribosomes
Ribosomes are the cellular structures responsible for protein synthesis.
When viewed through an electron microscope, free ribosomes appear as
either clusters or single tiny dots floating freely in the cytoplasm.
Ribosomes may be attached to either the cytoplasmic side of the plasma
membrane or the cytoplasmic side of the endoplasmic reticulum. Electron
microscopy has shown that ribosomes consist of large and small subunits.
Ribosomes are enzyme complexes that are responsible for protein synthesis.
Because protein synthesis is essential for all cells, ribosomes are found in
practically every cell, although they are smaller in prokaryotic cells. They
are particularly abundant in immature red blood cells for the synthesis of
hemoglobin, which functions in the transport of oxygen throughout the
body.
Mitochondria
Mitochondria (singular = mitochondrion) are often called the
“powerhouses” or “energy factories” of a cell because they are responsible
for making adenosine triphosphate (ATP), the cell’s main energy-carrying
molecule. The formation of ATP from the breakdown of glucose is known
as cellular respiration. Mitochondria are oval-shaped, double-membrane
organelles ({link]) that have their own ribosomes and DNA. Each
membrane is a phospholipid bilayer embedded with proteins. The inner
layer has folds called cristae, which increase the surface area of the inner
membrane. The area surrounded by the folds is called the mitochondrial
matrix. The cristae and the matrix have different roles in cellular
respiration.
In keeping with our theme of form following function, it is important to
point out that muscle cells have a very high concentration of mitochondria
because muscle cells need a lot of energy to contract.
Mitochondrial
matrix
Cristae
Outer
membrane
Inner
membrane
This transmission electron micrograph shows a
mitochondrion as viewed with an electron
microscope. Notice the inner and outer
membranes, the cristae, and the mitochondrial
matrix. (credit: modification of work by Matthew
Britton; scale-bar data from Matt Russell)
Section Summary
Like a prokaryotic cell, a eukaryotic cell has a plasma membrane,
cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a
prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a
membrane), and has other membrane-bound organelles that allow for
compartmentalization of functions. The plasma membrane is a phospholipid
bilayer embedded with proteins. The nucleolus within the nucleus is the site
for ribosome assembly. Ribosomes are found in the cytoplasm or are
attached to the cytoplasmic side of the plasma membrane or endoplasmic
reticulum. They perform protein synthesis. Mitochondria perform cellular
respiration and produce ATP. Vesicles are storage and transport
compartments.
The endomembrane system includes the nuclear envelope, the endoplasmic
reticulum, Golgi apparatus, lysosomes, vesicles, as well as the plasma
membrane. These cellular components work together to modify, package,
tag, and transport membrane lipids and proteins.
Art Connections
Exercise:
Problem:
[link] Why does the cis face of the Golgi not face the plasma
membrane?
Solution:
[link] Because that face receives chemicals from the ER, which is
toward the center of the cell.
Multiple Choice
Exercise:
Problem:
Which of the following is found both in eukaryotic and prokaryotic
cells?
a. nucleus
b. mitochondrion
c. vessicle
d. ribosome
Solution:
D
Exercise:
Problem:
Which of the following is not a component of the endomembrane
system?
a. mitochondrion
b. Golgi apparatus
c. endoplasmic reticulum
d. lysosome
Solution:
A
Free Response
Exercise:
Problem:
In the context of cell biology, what do we mean by form follows
function? What are at least two examples of this concept?
Solution:
“Form follows function” refers to the idea that the function of a body
part dictates the form of that body part. As an example, organisms like
birds or fish that fly or swim quickly through the air or water have
streamlined bodies that reduce drag. At the level of the cell, in tissues
involved in secretory functions, such as the salivary glands, the cells
have abundant Golgi.
Glossary
cell wall
a rigid cell covering made of cellulose in plants, peptidoglycan in
bacteria, non-peptidoglycan compounds in Archaea, and chitin in fungi
that protects the cell, provides structural support, and gives shape to
the cell
central vacuole
a large plant cell organelle that acts as a storage compartment, water
reservoir, and site of macromolecule degradation
chloroplast
a plant cell organelle that carries out photosynthesis
cilium
(plural: cilia) a short, hair-like structure that extends from the plasma
membrane in large numbers and is used to move an entire cell or move
substances along the outer surface of the cell
cytoplasm
the entire region between the plasma membrane and the nuclear
envelope, consisting of organelles suspended in the gel-like cytosol,
the cytoskeleton, and various chemicals
cytoskeleton
the network of protein fibers that collectively maintains the shape of
the cell, secures some organelles in specific positions, allows
cytoplasm and vesicles to move within the cell, and enables unicellular
organisms to move
cytosol
the gel-like material of the cytoplasm in which cell structures are
suspended
desmosome
a linkage between adjacent epithelial cells that forms when cadherins
in the plasma membrane attach to intermediate filaments
endomembrane system
the group of organelles and membranes in eukaryotic cells that work
together to modify, package, and transport lipids and proteins
endoplasmic reticulum (ER)
a series of interconnected membranous structures within eukaryotic
cells that collectively modify proteins and synthesize lipids
extracellular matrix
the material, primarily collagen, glycoproteins, and proteoglycans,
secreted from animal cells that holds cells together as a tissue, allows
cells to communicate with each other, and provides mechanical
protection and anchoring for cells in the tissue
flagellum
(plural: flagella) the long, hair-like structure that extends from the
plasma membrane and is used to move the cell
gap junction
a channel between two adjacent animal cells that allows ions,
nutrients, and other low-molecular weight substances to pass between
the cells, enabling the cells to communicate
Golgi apparatus
a eukaryotic organelle made up of a series of stacked membranes that
sorts, tags, and packages lipids and proteins for distribution
lysosome
an organelle in an animal cell that functions as the cell’s digestive
component; it breaks down proteins, polysaccharides, lipids, nucleic
acids, and even worn-out organelles
mitochondria
(singular: mitochondrion) the cellular organelles responsible for
carrying out cellular respiration, resulting in the production of ATP, the
cell’s main energy-carrying molecule
nuclear envelope
the double-membrane structure that constitutes the outermost portion
of the nucleus
nucleolus
the darkly staining body within the nucleus that is responsible for
assembling ribosomal subunits
nucleus
the cell organelle that houses the cell’s DNA and directs the synthesis
of ribosomes and proteins
peroxisome
a small, round organelle that contains hydrogen peroxide, oxidizes
fatty acids and amino acids, and detoxifies many poisons
plasma membrane
a phospholipid bilayer with embedded (integral) or attached
(peripheral) proteins that separates the internal contents of the cell
from its surrounding environment
plasmodesma
(plural: plasmodesmata) a channel that passes between the cell walls of
adjacent plant cells, connects their cytoplasm, and allows materials to
be transported from cell to cell
ribosome
a cellular structure that carries out protein synthesis
rough endoplasmic reticulum (RER)
the region of the endoplasmic reticulum that is studded with ribosomes
and engages in protein modification
smooth endoplasmic reticulum (SER)
the region of the endoplasmic reticulum that has few or no ribosomes
on its cytoplasmic surface and synthesizes carbohydrates, lipids, and
steroid hormones; detoxifies chemicals like pesticides, preservatives,
medications, and environmental pollutants, and stores calcium ions
tight junction
a firm seal between two adjacent animal cells created by protein
adherence
vacuole
a membrane-bound sac, somewhat larger than a vesicle, that functions
in cellular storage and transport
vesicle
a small, membrane-bound sac that functions in cellular storage and
transport; its membrane is capable of fusing with the plasma
membrane and the membranes of the endoplasmic reticulum and Golgi
apparatus
A More Detailed Look At The Cell Membrane
By the end of this section, you will be able to:
e Understand the fluid mosaic model of membranes
e Describe the functions of phospholipids, proteins, and carbohydrates in
membranes
A cell’s plasma membrane defines the boundary of the cell and determines
the nature of its contact with the environment. Cells exclude some
substances, take in others, and excrete still others, all in controlled
quantities. Plasma membranes enclose the borders of cells, but rather than
being a static bag, they are dynamic and constantly in flux. The plasma
membrane must be sufficiently flexible to allow certain cells, such as red
blood cells and white blood cells, to change shape as they pass through
narrow capillaries. These are the more obvious functions of a plasma
membrane. In addition, the surface of the plasma membrane carries markers
that allow cells to recognize one another, which is vital as tissues and
organs form during early development, and which later plays a role in the
“self” versus “non-self” distinction of the immune response.
The plasma membrane also carries receptors, which are attachment sites for
specific substances that interact with the cell. Each receptor is structured to
bind with a specific substance. For example, surface receptors of the
membrane create changes in the interior, such as changes in enzymes of
metabolic pathways. These metabolic pathways might be vital for providing
the cell with energy, making specific substances for the cell, or breaking
down cellular waste or toxins for disposal. Receptors on the plasma
membrane’s exterior surface interact with hormones or neurotransmitters,
and allow their messages to be transmitted into the cell. Some recognition
sites are used by viruses as attachment points. Although they are highly
specific, pathogens like viruses may evolve to exploit receptors to gain
entry to a cell by mimicking the specific substance that the receptor is
meant to bind. This specificity helps to explain why human
immunodeficiency virus (HIV) or any of the five types of hepatitis viruses
invade only specific cells.
Fluid Mosaic Model
In 1972, S. J. Singer and Garth L. Nicolson proposed a new model of the
plasma membrane that, compared to earlier understanding, better explained
both microscopic observations and the function of the plasma membrane.
This was called the fluid mosaic model. The model has evolved somewhat
over time, but still best accounts for the structure and functions of the
plasma membrane as we now understand them. The fluid mosaic model
describes the structure of the plasma membrane as a mosaic of components
—including phospholipids, cholesterol, proteins, and carbohydrates—in
which the components are able to flow and change position, while
maintaining the basic integrity of the membrane. Both phospholipid
molecules and embedded proteins are able to diffuse rapidly and laterally in
the membrane. The fluidity of the plasma membrane is necessary for the
activities of certain enzymes and transport molecules within the membrane.
Plasma membranes range from 5—10 nanometers (nm) thick. As a
comparison, human red blood cells, visible via light microscopy, are
approximately 8 micrometers (yim) thick, or approximately 1,000 times
thicker than a plasma membrane. ({Llink])
Glycoprotein: protein with Glycolipid: lipid with
i | 7 carbohydrate
attached
carbohydrate attached
Peripheral membrane Phospholipid
bilayer
protein
Integral membrane Cholesterol
protein Protein channel
Filaments of the cytoskeleton
The fluid mosaic model of the plasma membrane
structure describes the plasma membrane as a fluid
combination of phospholipids, cholesterol,
proteins, and carbohydrates.
The plasma membrane is made up primarily of a bilayer of phospholipids
with embedded proteins, carbohydrates, glycolipids, and glycoproteins, and,
in animal cells, cholesterol. The amount of cholesterol in animal plasma
membranes regulates the fluidity of the membrane and changes based on
the temperature of the cell’s environment. In other words, cholesterol acts
as antifreeze in the cell membrane and is more abundant in animals that live
in cold climates.
The main fabric of the membrane is composed of two layers of
phospholipid molecules, and the polar ends of these molecules (which look
like a collection of balls in an artist’s rendition of the model) ({link]) are in
contact with aqueous fluid both inside and outside the cell. Thus, both
surfaces of the plasma membrane are hydrophilic. In contrast, the interior of
the membrane, between its two surfaces, is a hydrophobic or nonpolar
region because of the fatty acid tails. This region has no attraction for water
or other polar molecules.
Proteins make up the second major chemical component of plasma
membranes. Integral proteins are embedded in the plasma membrane and
may span all or part of the membrane. Integral proteins may serve as
channels or pumps to move materials into or out of the cell. Peripheral
proteins are found on the exterior or interior surfaces of membranes,
attached either to integral proteins or to phospholipid molecules. Both
integral and peripheral proteins may serve as enzymes, as structural
attachments for the fibers of the cytoskeleton, or as part of the cell’s
recognition sites.
Carbohydrates are the third major component of plasma membranes. They
are always found on the exterior surface of cells and are bound either to
proteins (forming glycoproteins) or to lipids (forming glycolipids). These
carbohydrate chains may consist of 2-60 monosaccharide units and may be
either straight or branched. Along with peripheral proteins, carbohydrates
form specialized sites on the cell surface that allow cells to recognize each
other.
Note:
Evolution in Action
How Viruses Infect Specific Organs
Specific glycoprotein molecules exposed on the surface of the cell
membranes of host cells are exploited by many viruses to infect specific
organs. For example, HIV is able to penetrate the plasma membranes of
specific kinds of white blood cells called T-helper cells and monocytes, as
well as some cells of the central nervous system. The hepatitis virus attacks
only liver cells.
These viruses are able to invade these cells, because the cells have binding
sites on their surfaces that the viruses have exploited with equally specific
glycoproteins in their coats. ({link]). The cell is tricked by the mimicry of
the virus coat molecules, and the virus is able to enter the cell. Other
recognition sites on the virus’s surface interact with the human immune
system, prompting the body to produce antibodies. Antibodies are made in
response to the antigens (or proteins associated with invasive pathogens).
These same sites serve as places for antibodies to attach, and either destroy
or inhibit the activity of the virus. Unfortunately, these sites on HIV are
encoded by genes that change quickly, making the production of an
effective vaccine against the virus very difficult. The virus population
within an infected individual quickly evolves through mutation into
different populations, or variants, distinguished by differences in these
recognition sites. This rapid change of viral surface markers decreases the
effectiveness of the person’s immune system in attacking the virus, because
the antibodies will not recognize the new variations of the surface patterns.
Cytoplasm
HIV docks at and binds to the CD4
receptor, a glycoprotein on the surface
of T cells, before entering, or
infecting, the cell. (credit:
modification of work by US National
Institutes of Health/National Institute
of Allergy and Infectious Diseases)
Section Summary
The modern understanding of the plasma membrane is referred to as the
fluid mosaic model. The plasma membrane is composed of a bilayer of
phospholipids, with their hydrophobic, fatty acid tails in contact with each
other. The landscape of the membrane is studded with proteins, some of
which span the membrane. Some of these proteins serve to transport
materials into or out of the cell. Carbohydrates are attached to some of the
proteins and lipids on the outward-facing surface of the membrane. These
form complexes that function to identify the cell to other cells. The fluid
nature of the membrane owes itself to the configuration of the fatty acid
tails, the presence of cholesterol embedded in the membrane (in animal
cells), and the mosaic nature of the proteins and protein-carbohydrate
complexes, which are not firmly fixed in place. Plasma membranes enclose
the borders of cells, but rather than being a static bag, they are dynamic and
constantly in flux.
Multiple Choice
Exercise:
Problem:
Which plasma membrane component can be either found on its surface
or embedded in the membrane structure?
a. protein
b. cholesterol
c. carbohydrate
d. phospholipid
Solution:
A
Exercise:
Problem:
The tails of the phospholipids of the plasma membrane are composed
of and are
a. phosphate groups; hydrophobic
b. fatty acid groups; hydrophilic
c. phosphate groups; hydrophilic
d. fatty acid groups; hydrophobic
Solution:
D
Free Response
Exercise:
Problem:
Why is it advantageous for the cell membrane to be fluid in nature?
Solution:
The fluidity of the cell membrane is necessary for the operation of
some enzymes and transport mechanisms within the membrane.
Glossary
fluid mosaic model
a model of the structure of the plasma membrane as a mosaic of
components, including phospholipids, cholesterol, proteins, and
glycolipids, resulting in a fluid rather than static character
Passive Transport Mechanisms
By the end of this section, you will be able to:
e Explain why and how passive transport occurs
e Understand the processes of simple diffusion, facilitated diffusion, and
osmosis
¢ Define tonicity and describe its relevance to passive transport
e Predict changes in osmosis and cell shape on the basis of tonicity
differences
Plasma membranes must allow certain substances to enter and leave a cell,
while preventing harmful material from entering and essential material from
leaving. In other words, plasma membranes are selectively permeable—
they allow some substances through but not others. If they were to lose this
selectivity, the cell would no longer be able to sustain itself, and it would be
destroyed. Some cells require larger amounts of specific substances than do
other cells; they must have a way of obtaining these materials from the
extracellular fluids. This may happen passively, as certain materials move
back and forth, or the cell may have special mechanisms that ensure
transport. Most cells expend most of their energy, in the form of adenosine
triphosphate (ATP), to create and maintain an uneven distribution of ions on
the opposite sides of their membranes. The structure of the plasma
membrane contributes to these functions, but it also presents some
problems.
The most direct forms of membrane transport are passive. Passive
transport is a naturally occurring phenomenon and does not require the cell
to expend energy to accomplish the movement. In passive transport,
substances move from an area of higher concentration to an area of lower
concentration in a process called diffusion. Concentration refers to the
amount of a solute in a volume of solution. The greater the amount of solute
in the volume, the higher the concentration. A physical space in which there
is a different concentration of a single substance is said to have a
concentration gradient. For example, if a drop of food coloring is added to
a glass of water, the place where the dye lands represents a high solute
concentration and the rest of the water represents a low solute
concentration. Over time, the dye will passively move via diffusion until the
concentration is equal throughout the water.
Selective Permeability
Recall that plasma membranes have hydrophilic and hydrophobic regions.
This characteristic helps the movement of certain materials through the
membrane and hinders the movement of others. Lipid-soluble material can
easily slip through the hydrophobic lipid core of the membrane. Substances
such as the fat-soluble vitamins A, D, E, and K readily pass through the
plasma membranes in the digestive tract and other tissues. Fat-soluble drugs
also gain easy entry into cells and are readily transported into the body’s
tissues and organs. Molecules of oxygen and carbon dioxide, which are
relative small and nonpolar, pass through by simple diffusion.
Polar substances, with the exception of water, present problems for the
membrane. While some polar molecules connect easily with the outside of a
cell, they cannot readily pass through the lipid core of the plasma
membrane. Additionally, whereas small ions could easily slip through the
spaces in the mosaic of the membrane, their charge prevents them from
doing so. Ions such as sodium, potassium, calcium, and chloride must have
a special means of penetrating plasma membranes. Simple sugars and
amino acids also need help with transport across plasma membranes.
Diffusion
Diffusion is a passive process of transport. A single substance tends to
move from an area of high concentration to an area of low concentration
until the concentration is equal across the space. You are familiar with
diffusion of substances through the air. For example, think about someone
opening a bottle of perfume in a room filled with people. The perfume is at
its highest concentration in the bottle and is at its lowest at the edges of the
room. The perfume vapor will diffuse, or spread away, from the bottle, and
gradually, more and more people will smell the perfume as it spreads.
Materials move within the cell’s cytosol by diffusion, and certain materials
move through the plasma membrane by diffusion ([link]). Diffusion
expends no energy. Rather the different concentrations of materials in
different areas are a form of potential energy, and diffusion is the
dissipation of that potential energy as materials move down their
concentration gradients, from high to low.
Lipid bilayer
(plasma {
membrane)
TIME
Diffusion through a permeable membrane follows the
concentration gradient of a substance, moving the substance from
an area of high concentration to one of low concentration. When
the concentrations are equal on both sides of the membrane, a
concentration gradient no longer exists and there is no longer net
movement of solution in either direction.(credit: modification of
work by Mariana Ruiz Villarreal)
Each separate substance in a medium, such as the extracellular fluid, has its
own concentration gradient, independent of the concentration gradients of
other materials. Additionally, each substance will diffuse according to that
gradient.
Several factors affect the rate of diffusion:
e Extent of the concentration gradient: The greater the difference in
concentration, the more rapid the diffusion. The closer the distribution
of the material gets to equal on both sides, the slower the rate of
diffusion becomes.
e Mass of the molecules diffusing: More massive molecules move more
slowly, because it is more difficult for them to move between the
molecules of the substance they are moving through; therefore, they
diffuse more slowly.
e Temperature: Higher temperatures increase the energy and therefore
the movement of the molecules, increasing the rate of diffusion.
Note:
Concept in Action
east
Hi:
wees, OPenstax COLLEGE”
ite = '
ee o
For an animation of the diffusion process in action, view this short video
on cell membrane transport.
Facilitated transport
In facilitated transport, also called facilitated diffusion, material moves
across the plasma membrane with the assistance of transmembrane proteins
down a concentration gradient (from high to low concentration) without the
expenditure of cellular energy. However, the substances that undergo
facilitated transport would otherwise not diffuse easily or quickly across the
plasma membrane. The solution to moving polar substances and other
substances across the plasma membrane rests in the proteins that span its
surface. The material being transported is first attached to protein or
glycoprotein receptors on the exterior surface of the plasma membrane.
This allows the material that is needed by the cell to be removed from the
extracellular fluid. The substances are then passed to specific integral
proteins that facilitate their passage, because they form channels or pores
that allow certain substances to pass through the membrane. The integral
proteins involved in facilitated transport are collectively referred to as
transport proteins, and they function as either channels for the material or
carriers.
Osmosis
Osmosis is the diffusion of water through a semipermeable membrane
according to the concentration gradient of water across the membrane.
Whereas diffusion transports material across membranes and within cells,
osmosis transports only water across a membrane and the membrane limits
the diffusion of solutes in the water. Osmosis is a special case of diffusion.
Water, like other substances, moves from an area of higher concentration to
one of lower concentration. Imagine a beaker with a semipermeable
membrane, separating the two sides or halves ({link]). On both sides of the
membrane, the water level is the same, but there are different
concentrations on each side of a dissolved substance, or solute, that cannot
cross the membrane. If the volume of the water is the same, but the
concentrations of solute are different, then there are also different
concentrations of water, the solvent, on either side of the membrane.
Semipermeable membrane
In osmosis, water always moves from an
area of higher concentration (of water) to
one of lower concentration (of water). In this
system, the solute cannot pass through the
selectively permeable membrane due to its
size.
A principle of diffusion is that the molecules move around and will spread
evenly throughout the medium if they can. However, only the material
capable of getting through the membrane will diffuse through it. In this
example, the solute cannot diffuse through the membrane, but the water
can. Water has a concentration gradient in this system. Therefore, water will
diffuse down its concentration gradient, crossing the membrane to the side
where it is less concentrated. This diffusion of water through the membrane
—osmosis—will continue until the concentration gradient of water goes to
zero. Osmosis proceeds constantly in living systems.
Note:
Concept in Action
Oa eo
—"
meee OPENStAX COLLEGE”
Watch this video that illustrates diffusion in hot versus cold solutions.
Tonicity
Tonicity describes the amount of solute in a solution. Three terms—
hypotonic, isotonic, and hypertonic—are used to relate the concentration of
solutes inside of a cell compared to the concentration of solutes in the fluid
that contains the cells. In a hypotonic solution, such as tap water, the
extracellular fluid has a lower concentration of solutes than the fluid inside
the cell, and water enters the cell. (In living systems, the point of reference
is always the cytoplasm, so the prefix hypo- means that the extracellular
fluid has a lower concentration of solutes, than the cell cytoplasm.) It also
means that the extracellular fluid has a higher concentration of water than
does the cell. In this situation, water will follow its concentration gradient
and enter the cell. This may cause an animal cell to burst, or lyse. Note in
these cases that there is always a net movement of water (the solvent)
towards the hypertonic solution by the process of osmosis. That is because
the hypertonic solution has a lower concentration of water and the solute
can't pass through the membrane.
In a hypertonic solution (the prefix hyper- refers to the extracellular fluid
having a higher concentration of solutes than the cell’s cytoplasm), the fluid
contains less water than the cell does, such as seawater. Because the cell has
a lower concentration of solutes, the water will leave the cell. In effect, the
solute is drawing the water out of the cell. This may cause an animal cell to
shrivel, or crenate.
In an isotonic solution, the extracellular fluid has the same osmolarity as
the cell. If the concentration of solutes of the cell matches that of the
extracellular fluid, there will be no net movement of water into or out of the
cell. Blood cells in hypertonic, isotonic, and hypotonic solutions take on
characteristic appearances ({link]).
Note:
Art Connection
Hypertonic Isotonic Hypotonic
solution solution solution
1.0 > >
Osmotic pressure changes the shape of red blood
cells in hypertonic, isotonic, and hypotonic
solutions. Note the tonicity terms refer to the
solution containing the cells versus the solution
inside the cell. If a cell is immersed in a hypotonic
solution, then the contents of the cell are
hypertonic and there is a net movement of water
into the cell.(credit: modification of work by
Mariana Ruiz Villarreal)
Section Summary
The passive forms of transport, diffusion and osmosis, move material of
small molecular weight. Substances diffuse from areas of high
concentration to areas of low concentration, and this process continues until
the substance is evenly distributed in a system. In solutions of more than
one substance, each type of molecule diffuses according to its own
concentration gradient. Many factors can affect the rate of diffusion,
including concentration gradient, the sizes of the particles that are diffusing,
and the temperature of the system.
In living systems, diffusion of substances into and out of cells is mediated
by the plasma membrane. Some materials diffuse readily through the
membrane, but others are hindered, and their passage is only made possible
by protein channels and carriers. The chemistry of living things occurs in
aqueous solutions, and balancing the concentrations of those solutions is an
ongoing problem. In living systems, diffusion of some substances would be
slow or difficult without membrane proteins.
Art Connections
Exercise:
Problem:
[link] A doctor injects a patient with what he thinks is isotonic saline
solution. The patient dies, and autopsy reveals that many red blood
cells have been destroyed. Do you think the solution the doctor
injected was really isotonic?
Solution:
[link] No, it must have been hypotonic, as a hypotonic solution would
cause water to enter the cells, thereby making them burst.
Multiple Choice
Exercise:
Problem: Water moves via osmosis
a. throughout the cytoplasm
b. from an area with a high concentration of other solutes to a lower
one
c. from an area with a low concentration of solutes to an area with a
higher one
d. from an area with a low concentration of water to one of higher
concentration
Solution:
C
Exercise:
Problem:
The principal force driving movement in diffusion is
a. temperature
b. particle size
c. concentration gradient
d. membrane surface area
Free Response
Exercise:
Problem: Why does osmosis occur?
Solution:
Water moves through a semipermeable membrane in osmosis because
there is a concentration gradient across the membrane of solute and
solvent. The solute cannot effectively move to balance the
concentration on both sides of the membrane, so water moves to
achieve this balance.
Glossary
concentration gradient
an area of high concentration across from an area of low concentration
diffusion
a passive process of transport of low-molecular weight material down
its concentration gradient
facilitated transport
a process by which material moves down a concentration gradient
(from high to low concentration) using integral membrane proteins
hypertonic
describes a solution in which extracellular fluid has higher osmolarity
than the fluid inside the cell
hypotonic
describes a solution in which extracellular fluid has lower osmolarity
than the fluid inside the cell
isotonic
describes a solution in which the extracellular fluid has the same
osmolarity as the fluid inside the cell
osmolarity
the total amount of substances dissolved in a specific amount of
solution
osmosis
the transport of water through a semipermeable membrane from an
area of high water concentration to an area of low water concentration
across a membrane
passive transport
a method of transporting material that does not require energy
selectively permeable
the characteristic of a membrane that allows some substances through
but not others
solute
a substance dissolved in another to form a solution
tonicity
the amount of solute in a solution.
Active Transport Mechanisms
By the end of this section, you will be able to:
e Describe how active transport can use ATP energy to move solutes
against the concentration gradient
e Describe the general process of endocytosis, including phagocytosis
e Describe the general process of exocytosis
Active transport mechanisms require the use of the cell’s energy, usually
in the form of adenosine triphosphate (ATP). If a substance must move into
the cell against its concentration gradient, that is, if the concentration of the
substance inside the cell must be greater than its concentration in the
extracellular fluid, the cell must use energy to move the substance. Some
active transport mechanisms move small-molecular weight material, such as
ions, through the membrane.
In addition to moving small ions and molecules through the membrane,
cells also need to remove and take in larger molecules and particles. Some
cells are even capable of engulfing entire unicellular microorganisms. You
might have correctly hypothesized that the uptake and release of large
particles by the cell requires energy. A large particle, however, cannot pass
through the membrane, even with energy supplied by the cell.
Primary Active Transport
There are several types of active transport. The principle one that will be
discussed is primary active transport, which uses a combination of ATP
energy and a transport protein to move substances across the membrane
against the concentration gradient. ATP is hydrolyzed, via an enzyme-
catalyzed reaction, to ADP and the lost phosphate group attaches to the
protein. This joining causes a conformational change in the shape of the
transport protein and the particular substance is moved across the
membrane against the concentration gradient. An example of primary active
transport is the sodium-potassium pump, which is involved in nerve
impulses and is discussed in a later chapter.
Endocytosis
Endocytosis is a type of active transport that moves particles, such as large
molecules, parts of cells, and even whole cells, into a cell. There are
different variations of endocytosis, but all share a common characteristic:
The plasma membrane of the cell invaginates, forming a pocket around the
target particle. The pocket pinches off, resulting in the particle being
contained in a newly created vacuole that is formed from the plasma
membrane.
Phagocytosis Pinocytosis Receptor-mediated
endocytosis
@
s
a . *
rs Large particle *
2 . ~ " *
A
Plasma
membrane
Clathrin Receptor
©
Vacuole
©...
Coated vesicle
(a) (b) (°c)
Three variations of endocytosis are shown. (a) In one form of
endocytosis, phagocytosis, the cell membrane surrounds the
particle and pinches off to form an intracellular vacuole. (b) In
another type of endocytosis, pinocytosis, the cell membrane
surrounds a small volume of fluid and pinches off, forming a
vesicle. (c) In receptor-mediated endocytosis, uptake of substances
by the cell is targeted to a single type of substance that binds at the
receptor on the external cell membrane. (credit: modification of
work by Mariana Ruiz Villarreal)
Phagocytosis is the process by which large particles, such as cells, are
taken in by a cell. For example, when microorganisms invade the human
body, a type of white blood cell called a neutrophil removes the invader
through this process, surrounding and engulfing the microorganism, which
is then destroyed by the neutrophil ((link]).
Exocytosis
In contrast to these methods of moving material into a cell is the process of
exocytosis. Exocytosis is the opposite of the processes discussed above in
that its purpose is to expel material from the cell into the extracellular fluid.
A particle enveloped in membrane fuses with the interior of the plasma
membrane. This fusion opens the membranous envelope to the exterior of
the cell, and the particle is expelled into the extracellular space ([link]).
Exocytosis
Extracellular fluid
SS Cyctoplasm
Vesicle
In exocytosis, a vesicle
migrates to the plasma
membrane, binds, and
releases its contents to the
outside of the cell. (credit:
modification of work by
Mariana Ruiz Villarreal)
Section Summary
Primary active transport uses energy stored in ATP to fuel the transport.
Active transport of small molecular-size material uses integral proteins in
the cell membrane to move the material—these proteins are analogous to
pumps. Some pumps, which carry out primary active transport, couple
directly with ATP to drive their action.
Endocytosis methods require the direct use of ATP to fuel the transport of
large particles such as macromolecules; parts of cells or whole cells can be
engulfed by other cells in a process called phagocytosis. In phagocytosis, a
portion of the membrane invaginates and flows around the particle,
eventually pinching off and leaving the particle wholly enclosed by an
envelope of plasma membrane. The cell expels waste and other particles
through the reverse process, exocytosis. Wastes are moved outside the cell,
pushing a membranous vesicle to the plasma membrane, allowing the
vesicle to fuse with the membrane and incorporating itself into the
membrane structure, releasing its contents to the exterior of the cell.
Multiple Choice
Exercise:
Problem:
Active transport must function continuously because
a. plasma membranes wear out
b. cells must be in constant motion
c. facilitated transport opposes active transport
d. diffusion is constantly moving the solutes in the other direction
Solution:
D
Free Response
Exercise:
Problem:
Where does the cell get energy for active transport processes?
Solution:
The cell harvests energy from ATP produced by its own metabolism to
power active transport processes, such as pumps.
Glossary
active transport
the method of transporting material that requires energy
electrochemical gradient
a gradient produced by the combined forces of the electrical gradient
and the chemical gradient
endocytosis
a type of active transport that moves substances, including fluids and
particles, into a cell
exocytosis
a process of passing material out of a cell
phagocytosis
a process that takes macromolecules that the cell needs from the
extracellular fluid; a variation of endocytosis
pinocytosis
a process that takes solutes that the cell needs from the extracellular
fluid; a variation of endocytosis
receptor-mediated endocytosis
a variant of endocytosis that involves the use of specific binding
proteins in the plasma membrane for specific molecules or particles
Introduction to the Central Dogma of Molecular Biology
class="introduction"
This is
Dolly, the
first sheep
produced
using a
novel type
of molecular
genetic
technology
that
involved
transfer of
the nucleus
from an
adult udder
cell to an
unfertilized
egg whose
nucleus had
been
removed.
This egg
was then
transplanted
into another
female
sheep to
undergo
developmen
t during
pregnancy.
Sheep, as well as humans, normally begin life as a single cell called a
fertilized egg or zygote. From this one cell, trillions of cells will ultimately
be derived through the process of cell division. Prior to each cell division
event, the DNA must replicate. Also, cells must produce proteins needed to
accomplish specific functions. These events are described by the central
dogma of molecular biology, which states that DNA contains information to
replicate itself and that specific regions of the DNA (called genes) contains
the information needed to make RNA, which is in turn used to produce
needed proteins. In this chapter, we will learn more about the steps of these
processes.
DNA and RNA
By the end of this section, you will be able to:
¢ Describe the structures of DNA and RNA
e Describe how eukaryotic DNA is arranged in the cell
In the 1950s, Francis Crick and James Watson worked together at the
University of Cambridge, England, to determine the structure of DNA.
Other scientists, such as Linus Pauling and Maurice Wilkins, were also
actively exploring this field. Pauling had discovered the secondary structure
of proteins using X-ray crystallography. X-ray crystallography is a method
for investigating molecular structure by observing the patterns formed by
X-rays shot through a crystal of the substance. The patterns give important
information about the structure of the molecule of interest. In Wilkins’ lab,
researcher Rosalind Franklin was using X-ray crystallography to understand
the structure of DNA. Watson and Crick were able to piece together the
puzzle of the DNA molecule using Franklin's data ({link]). Watson and
Crick also had key pieces of information available from other researchers
such as Chargaff’s rules. Chargaff had shown that of the four kinds of
monomers (nucleotides) present ina DNA molecule, two types were always
present in equal amounts and the remaining two types were also always
present in equal amounts. This meant they were always paired in some way.
In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded
the Nobel Prize in Medicine for their work in determining the structure of
DNA.
(b)
Pioneering scientists (a) James Watson and Francis Crick
are pictured here with American geneticist Maclyn
McCarty. Scientist Rosalind Franklin discovered (b) the
X-ray diffraction pattern of DNA, which helped to
elucidate its double helix structure. (credit a:
modification of work by Marjorie McCarty; b:
modification of work by NIH)
Now let’s consider the structure of the two types of nucleic acids,
deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The building
blocks of DNA are nucleotides, which are made up of three parts: a
deoxyribose (a 5-carbon sugar with the carbons designated as 1', 2', 3', 4'
and 5'), a phosphate group attached to the 5'carbon, and a nitrogenous
base attached to the 1'carbon([link]). There are four types of nitrogenous
bases in DNA. Adenine (A) and guanine (G) are double-ringed purines, and
cytosine (C) and thymine (T) are smaller, single-ringed pyrimidines. The
nucleotide is named according to the nitrogenous base it contains.
Pyrimidines
°
c A CH
3
NZ cH HN~ c~
iS e.
Ca Sn
H
(e)
Thymine
T
Purines
NH2
|
N (e
oe
HC. Il | HC, ll |
Ny
Guanine Adenine
G A
(a) Each DNA nucleotide is made up of a sugar, a
phosphate group, and a base. Four of the five carbon
atoms in the sugar are not shown to simplify the diagram;
each point on the ring is where a carbon (1' to 4') would
be would be located; the position of the 5' carbon is
above the ring at the point adjacent to an oxygen of the
phosphate group. (b) Cytosine and thymine are
pyrimidines. Guanine and adenine are purines. Note: As
the 2' carbon is attached to an -OH group, the sugar
shown is actually ribose, not deoxyribose as intended.
The phosphate group of one nucleotide bonds covalently with the sugar
molecule of the next nucleotide, and so on, forming a long polymer of
nucleotide monomers. The sugar—phosphate groups line up in a “backbone”
for each single strand of DNA, and the nucleotide bases stick out from this
backbone. The carbon atoms of the five-carbon sugar are numbered
clockwise from the oxygen as 1’, 2', 3', 4', and 5' (1' is read as “one prime”).
The phosphate group is attached to the 5' carbon of one nucleotide and the
3' carbon of the next nucleotide. In its natural state, each DNA molecule is
actually composed of two single strands held together along their length
with hydrogen bonds between the bases.
Watson and Crick proposed that the DNA is made up of two strands that are
twisted around each other to form a right-handed helix, called a double
helix. Base-pairing takes place between a purine and pyrimidine: namely, A
pairs with T, and G pairs with C. In other words, adenine and thymine are
complementary base pairs, and cytosine and guanine are also
complementary base pairs. This is the basis for Chargaff’s rule; because of
their complementarity, there is as much adenine as thymine ina DNA
molecule and as much guanine as cytosine. Adenine and thymine are
connected by two hydrogen bonds, and cytosine and guanine are connected
by three hydrogen bonds. The two strands are anti-parallel in nature; that is,
one strand will have the 3' carbon of the sugar in the “upward” position,
whereas the other strand will have the 5' carbon in the upward position. The
diameter of the DNA double helix is uniform throughout because a purine
(two rings) always pairs with a pyrimidine (one ring) and their combined
lengths are always equal. ({link]).
Hydrogen bonds
Nitrogenous bases: Thymine _--H2N
3 5° @==PAdenine
S
[== Thymine Os, ree rae . ©
—=—PGuanine -F O \=yj O ©
O
EEX Cytosine ON ( wp
1e) Oo *O
HoN
Adenine
oO, _O N ow
oe = a
Base pair © o NY NH y—N O86
sent Cytosine 0.
Sugar HO Guanine He 9 5
phosphate
backbone
Sugar-phosphate Bases Sugar-phosphate
3 5’ backbone backbone
(a) (b)
DNA (a) forms a double stranded helix, and (b)
adenine pairs with thymine and cytosine pairs with
guanine. (credit a: modification of work by Jerome
Walker, Dennis Myts)
The Structure of RNA
There is a second nucleic acid in all cells called ribonucleic acid, or RNA.
Like DNA, RNA is a polymer of nucleotides. Each of the nucleotides in
RNA is made up of a nitrogenous base, a five-carbon sugar, and a
phosphate group. In the case of RNA, the five-carbon sugar is ribose, not
deoxyribose. Ribose has a hydroxyl group at the 2' carbon, unlike
deoxyribose, which has only a hydrogen atom ([link]).
OH H
Ribose Deoxyribose
The difference between the ribose
found in RNA and the deoxyribose
found in DNA is that ribose has a
hydroxyl group at the 2' carbon.
RNA nucleotides contain the nitrogenous bases adenine, cytosine, and
guanine. However, they do not contain thymine, which is instead replaced
by uracil, symbolized by a “U.” RNA exists as a single-stranded molecule
rather than a double-stranded helix. Molecular biologists have named
several kinds of RNA on the basis of their function. These include
messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA
(rRNA)—molecules that are involved in the production of proteins from the
DNA code.
How DNA Is Arranged in the Cell
DNA is a working molecule; it must be replicated when a cell is ready to
divide, and it must be “read” to produce the molecules, such as proteins, to
carry out the functions of the cell. For this reason, the DNA is protected and
packaged in very specific ways. In addition, DNA molecules can be very
long. Stretched end-to-end, the DNA molecules in a single human cell
would come to a length of about 2 meters,or 6.5 feet. Thus, the DNA for a
cell must be packaged in a very ordered way to fit and function within a
structure (the cell) that is not visible to the naked eye. The chromosomes of
prokaryotes are much simpler than those of eukaryotes in many of their
features ({link]). Most prokaryotes contain a single, circular chromosome
that is found in an area in the cytoplasm called the nucleoid.
Nucleoid
(folded
chromosome)
Eukaryote Prokaryote
A eukaryote contains a well-defined nucleus,
whereas in prokaryotes, the chromosome lies in the
cytoplasm in an area called the nucleoid.
The size of the genome in one of the most well-studied prokaryotes,
Escherichia coli, is 4.6 million base pairs, which would extend a distance of
about 1.6 millimeters (0.06 inches) if stretched out. So how does this fit
inside a small bacterial cell? The DNA is twisted beyond the double helix in
what is known as supercoiling. Some proteins are known to be involved in
the supercoiling; other proteins and enzymes help in maintaining the
supercoiled structure.
Eukaryotes, whose chromosomes each consist of a linear DNA molecule,
employ a different type of packing strategy to fit their DNA inside the
nucleus ({link]). At the most basic level, DNA is wrapped around proteins
known as histones to form structures called nucleosomes. The DNA is
wrapped tightly around the histone core. This nucleosome is linked to the
next one by a short strand of DNA that is free of histones. This is also
known as the “beads on a string” structure; the nucleosomes are the “beads”
and the short lengths of DNA between them are the “string.” The
nucleosomes, with their DNA coiled around them, stack compactly onto
each other to form a 30-nanometers—wide fiber. This fiber is further coiled
into a thicker and more compact structure. At the metaphase stage of
mitosis, when the chromosomes are lined up in the center of the cell, the
chromosomes are at their most compacted. They are approximately 700
nanometers in width, and are found in association with scaffold proteins.
In interphase, the phase of the cell cycle between mitoses at which the
chromosomes are decondensed, eukaryotic chromosomes have two distinct
regions that can be distinguished by staining. There is a tightly packaged
region that stains darkly, and a less dense region. The darkly staining
regions usually contain genes that are not active, and are found in the
regions of the centromere and telomeres. The lightly staining regions
usually contain genes that are active, with DNA packaged around
nucleosomes but not further compacted.
Organization of Eukaryotic Chromosomes
DNA double
helix
DNA wrapped
around histone
Nucleosomes
coiled into a
chromatin
fiber
Further
condensation
of chromatin
Duplicated
chromosome
These figures illustrate the
compaction of the eukaryotic
chromosome.
Note:
Concept in Action
—
mess Openstax COLLEGE
Route E
De) age ee
Watch this animation of DNA packaging.
Section Summary
The model of the double-helix structure of DNA was proposed by Watson
and Crick. The DNA molecule is a polymer of nucleotides. Each nucleotide
is composed of a nitrogenous base, a five-carbon sugar (deoxyribose), and a
phosphate group. There are four nitrogenous bases in DNA, two purines
(adenine and guanine) and two pyrimidines (cytosine and thymine). A DNA
molecule is composed of two strands. Each strand is composed of
nucleotides bonded together covalently between the phosphate group of one
and the deoxyribose sugar of the next. From this backbone extend the bases.
The bases of one strand bond to the bases of the second strand with
hydrogen bonds. Adenine always bonds with thymine, and cytosine always
bonds with guanine. The bonding causes the two strands to spiral around
each other in a shape called a double helix. Ribonucleic acid (RNA) is a
second nucleic acid found in cells. RNA is a single-stranded polymer of
nucleotides. It also differs from DNA in that it contains the sugar ribose,
rather than deoxyribose, and the nucleotide uracil rather than thymine.
Various RNA molecules function in the process of forming proteins from
the genetic code in DNA.
Eukaryotes contain double-stranded linear DNA molecules packaged into
chromosomes. The DNA helix is wrapped around proteins to form
nucleosomes. The protein coils are further coiled, and during mitosis and
meiosis, the chromosomes become even more greatly coiled to facilitate
their movement.
Multiple Choice
Exercise:
Problem: Which of the following does cytosine pair with?
a. guanine
b. thymine
c. adenine
d. a pyrimidine
Solution:
A
Exercise:
Problem:
Prokaryotes contain a chromosome, and eukaryotes contain
chromosomes.
a. single-stranded circular; single-stranded linear
b. single-stranded linear; single-stranded circular
c. double-stranded circular; double-stranded linear
d. double-stranded linear; double-stranded circular
Solution:
C
Free Response
Exercise:
Problem: Describe the organization of the eukaryotic chromosome.
Solution:
The DNA is wound around proteins called histones. The histones then
stack together in a compact form that creates a fiber that is 30-nm
thick. The fiber is further coiled for greater compactness.
Exercise:
Problem:
Describe the structure and complementary base pairing of DNA.
Solution:
A single strand of DNA is a polymer of nucleotides joined covalently
between the phosphate group of one and the deoxyribose sugar of the
next to form a phosphodiester “backbone” from which the nitrogenous
bases stick out. In its natural state, DNA has two strands wound
around each other in a double helix. The bases on each strand are
bonded to each other with hydrogen bonds. Only specific bases bond
with each other; adenine bonds with thymine, and cytosine bonds with
guanine.
Glossary
deoxyribose
a five-carbon sugar molecule with a hydrogen atom rather than a
hydroxyl group in the 2' position; the sugar component of DNA
nucleotides
double helix
the molecular shape of DNA in which two strands of nucleotides wind
around each other in a spiral shape
nitrogenous base
a nitrogen-containing molecule that acts as a base; often referring to
one of the purine or pyrimidine components of nucleic acids
phosphate group
a molecular group consisting of a central phosphorus atom bound to
four oxygen atoms
The Basics of DNA Replication
By the end of this section, you will be able to:
e Explain the process of DNA replication
e Explain how mutations can cause an effect on the phenotype of an
organism.
When a cell divides, it is important that each daughter cell receives an
identical copy of the DNA. This is accomplished by the process of DNA
replication. The replication of DNA occurs during the synthesis phase, or S
phase, of the cell cycle, before the cell enters mitosis or meiosis.
The elucidation of the structure of the double helix provided a hint as to
how DNA is copied. Recall that adenine nucleotides pair with thymine
nucleotides, and cytosine with guanine. This means that the two strands are
complementary to each other. For example, a strand of DNA witha
nucleotide sequence of AGTCATGA will have a complementary strand
with the sequence TCAGTACT ((link]).
3!
LL LR) De
ee a ie ie a ee
ai
The two strands of DNA are
complementary, meaning the sequence of
bases in one strand can be used to create the
correct sequence of bases in the other strand.
Because of the complementarity of the two strands, having one strand
means that it is possible to recreate the other strand. This model for
replication suggests that the two strands of the double helix separate during
replication, and each strand serves as a template from which the new
complementary strand is copied ((Link]).
Semi-conservative model of DNA Replication
The semiconservative model of
DNA replication is shown. Gray
indicates the original DNA
strands, and blue indicates newly
synthesized DNA.
During DNA replication, each of the two strands that make up the double
helix serves as a template from which new strands are copied. The name of
the enzyme that copies the DNA is called DNA polymerase. It joins the
complementary nucleotides together to make up the new strand, which is
complementary to the parental or “old” strand. Each new double helix
consists of one parental strand and one new daughter strand. This is known
as semiconservative replication. When two DNA copies are formed, they
have an identical sequence of nucleotide bases and are divided equally into
two daughter cells during cell division. Therefore, each of the two daughter
cells has a complete copy of each chromosome.
DNA Repair
DNA polymerase can sometimes make an error by inserting a
noncomplementary base during DNA replication. Most mistakes are
quickly corrected; if they are not, they may result in a mutation—defined
as a permanent change in the DNA sequence. Mutations in genes may lead
to serious consequences because incorrect proteins are produced.
Section Summary
DNA replicates by a semi-conservative method in which each of the two
parental DNA strands act as a template for new DNA to be synthesized.
After replication, each DNA has one parental or “old” strand, and one
daughter or “new” strand.
Multiple Choice
Exercise:
Problem: DNA replicates by which of the following models?
a. conservative
b. semiconservative
c. dispersive
d. none of the above
Solution:
B
Free Response
Exercise:
Problem:
The sequence one strand of a DNA double helix is: *ATGGCTACAA
Beginning at the * end, what is the complimentary sequence?
Solution:
TACCGATGTT
Glossary
DNA ligase
the enzyme that catalyzes the joining of DNA fragments together
DNA polymerase
an enzyme that synthesizes a new strand of DNA complementary to a
template strand
helicase
an enzyme that helps to open up the DNA helix during DNA
replication by breaking the hydrogen bonds
lagging strand
during replication of the 3' to 5' strand, the strand that is replicated in
short fragments and away from the replication fork
leading strand
the strand that is synthesized continuously in the 5' to 3' direction that
is synthesized in the direction of the replication fork
mismatch repair
a form of DNA repair in which non-complementary nucleotides are
recognized, excised, and replaced with correct nucleotides
mutation
a permanent variation in the nucleotide sequence of a genome
nucleotide excision repair
a form of DNA repair in which the DNA molecule is unwound and
separated in the region of the nucleotide damage, the damaged
nucleotides are removed and replaced with new nucleotides using the
complementary strand, and the DNA strand is resealed and allowed to
rejoin its complement
Okazaki fragments
the DNA fragments that are synthesized in short stretches on the
lagging strand
primer
a short stretch of RNA nucleotides that is required to initiate
replication and allow DNA polymerase to bind and begin replication
replication fork
the Y-shaped structure formed during the initiation of replication
semiconservative replication
the method used to replicate DNA in which the double-stranded
molecule is separated and each strand acts as a template for a new
strand to be synthesized, so the resulting DNA molecules are
composed of one new strand of nucleotides and one old strand of
nucleotides
telomerase
an enzyme that contains a catalytic part and an inbuilt RNA template;
it functions to maintain telomeres at chromosome ends
telomere
the DNA at the end of linear chromosomes
Transcription
By the end of this section, you will be able to:
e Explain the central dogma
e Explain the main steps of transcription
e Describe how eukaryotic mRNA is processed
The second function of DNA (the first was replication) is to provide the
information needed to construct the proteins necessary so that the cell can
perform all of its functions. To do this, the DNA is “read” or transcribed
into an mRNA molecule. The mRNA then provides the code to form a
protein by a process called translation. Through the processes of
transcription and translation, a protein is built with a specific sequence of
amino acids that was originally encoded in the DNA. This module discusses
the details of transcription.
The Central Dogma of Molecular Biology: DNA Encodes RNA;
RNA Encodes Protein
The flow of genetic information in cells from DNA to mRNA to protein is
described by the central dogma ([link]), which states that genes specify the
sequences of mRNAs, which in turn specify the sequences of proteins.
Protein
The central dogma of molecular
biology states that DNA encodes
RNA, which in turn encodes
protein.
The copying of DNA to mRNA (i.e. transcription) is relatively
straightforward, with one nucleotide being added to the mRNA strand for
every complementary nucleotide read in the DNA strand. The translation to
protein is more complex because groups of three mRNA nucleotides
correspond to one amino acid of the protein sequence. However, as we shall
see in the next module, the translation to protein is still systematic, such that
nucleotides 1 to 3 correspond to amino acid 1, nucleotides 4 to 6 correspond
to amino acid 2, and so on. The groups of three nucleotides that specify an
amino acid are called codons.
Transcription: from DNA to mRNA
With the genes bound in the nucleus, transcription occurs in the nucleus of
the cell and the mRNA transcript must be transported to the cytoplasm.
Transcription occurs in three main stages: initiation, elongation, and
termination.
Initiation
Transcription requires the DNA double helix to partially unwind in the
region of mRNA synthesis. The region of unwinding is called a
transcription bubble. The DNA sequence onto which the proteins and
enzymes involved in transcription bind to initiate the process is called a
promoter. In most cases, promoters exist upstream of the genes they
regulate. The specific sequence of a promoter is very important because it
determines whether the corresponding gene is transcribed all of the time,
some of the time, or hardly at all ([link]).
DNA RNA
nontemplate
strand
Synthesis ——&,
RNA polymerase
DNA
template
strand Promoter
The initiation of transcription begins
when DNA is unwound, forming a
transcription bubble. Enzymes and
other proteins involved in
transcription bind at the promoter.
Elongation
Transcription always proceeds from one of the two DNA strands, which is
called the template strand. The mRNA product is complementary to the
template strand and is almost identical to the other DNA strand, called the
nontemplate strand, with the exception that RNA contains a uracil (U) in
place of the thymine (T) found in DNA. During elongation, an enzyme
called RNA polymerase proceeds along the DNA template adding
nucleotides by base pairing with the DNA template in a manner similar to
DNA replication, with the difference that an RNA strand is being
synthesized that does not remain bound to the DNA template. As elongation
proceeds, the DNA is continuously unwound ahead of the enzyme and
rewound behind it ([link]).
Nontemplate strand
G
GCACTC
ATGCCGCA 4
3’
G TACCACGTA
Oirany; 4
“Ection of Be eae 3
ye
o AI GGTGCAT
5
CS
Uy
TAcSSS¢o ts CACC CRO CAUS
3
TGecotcaGthS
5!
DNA RNA polymerase Template strand
During elongation, RNA polymerase tracks along the
DNA template, synthesizes mRNA in the 5' to 3
direction, and unwinds then rewinds the DNA as it is
read.
Termination
Once a gene is transcribed, the RNA polymerase needs to be instructed to
dissociate from the DNA template and liberate the newly made mRNA.
Depending on the gene being transcribed, there are two kinds of termination
signals, but both involve repeated nucleotide sequences in the DNA
template that result in RNA polymerase stalling, leaving the DNA template,
and freeing the mRNA transcript.
Eukaryotic RNA Processing
The newly transcribed eukaryotic mRNAs must undergo several processing
steps before they can be transferred from the nucleus to the cytoplasm and
translated into a protein.
The mRNA transcript is first coated in RNA-stabilizing proteins to prevent
it from degrading while it is processed and exported out of the nucleus. This
occurs while the pre-mRNA still is being synthesized by adding a special
nucleotide “cap” to the 5' end of the growing transcript. In addition to
preventing degradation, factors involved in protein synthesis recognize the
cap to help initiate translation by ribosomes.
Once elongation is complete, an enzyme then adds a string of
approximately 200 adenine residues to the 3' end, called the poly-A tail.
This modification further protects the pre-mRNA from degradation and
signals to cellular factors that the transcript needs to be exported to the
cytoplasm.
Eukaryotic genes are composed of protein-coding sequences called exons
(ex-on signifies that they are expressed) and intervening sequences called
introns (int-ron denotes their intervening role). Introns are removed from
the pre-mRNA during processing. Intron sequences in MRNA do not
encode functional proteins. It is essential that all of a pre-mRNA’s introns
be completely and precisely removed before protein synthesis so that the
exons join together to code for the correct amino acids. If the process errs
by even a single nucleotide, the sequence of the rejoined exons would be
shifted, and the resulting protein would be nonfunctional. The process of
removing introns and reconnecting exons is called splicing (({link]). Introns
are removed and degraded while the pre-mRNA is still in the nucleus.
Primary RNA transcript
u RNA processing
Spliced RNA
5' cap Poly-A tail
5’ untranslated 3’ untranslated
region region
Eukaryotic mRNA contains introns that
must be spliced out. A 5' cap and 3' tail
are also added.
Section Summary
mRNA synthesis is initiated at a promoter sequence on the DNA template.
Elongation synthesizes new mRNA (called a pre-mRNA). Termination
liberates the mRNA and occurs by mechanisms that stall the RNA
polymerase and cause it to fall off the DNA template. Newly transcribed
mRNAs are modified with a cap and a poly-A tail. These structures protect
the mature mRNA from degradation and help export it from the nucleus.
mRNAs also undergo splicing, in which introns are removed and exons are
reconnected with single-nucleotide accuracy. Only finished mRNAs are
exported from the nucleus to the cytoplasm.
Multiple Choice
Exercise:
Problem: A promoter is
a. a specific sequence of DNA nucleotides
b. a specific sequence of RNA nucleotides
c. a protein that binds to DNA
d. an enzyme that synthesizes RNA
Solution:
A
Exercise:
Problem:
Portions of eukaryotic MRNA sequence that are removed during RNA
processing are
da. exons
b. caps
c. poly-A tails
d. introns
Solution:
D
Glossary
exon
a sequence present in protein-coding mRNA after completion of pre-
mRNA splicing
intron
non—protein-coding intervening sequences that are spliced from
mRNA during processing
mRNA
messenger RNA; a form of RNA that carries the nucleotide sequence
code for a protein sequence that is translated into a polypeptide
sequence
nontemplate strand
the strand of DNA that is not used to transcribe mRNA; this strand is
identical to the mRNA except that T nucleotides in the DNA are
replaced by U nucleotides in the mRNA
promoter
a sequence on DNA to which RNA polymerase and associated factors
bind and initiate transcription
RNA polymerase
an enzyme that synthesizes an RNA strand from a DNA template
strand
splicing
the process of removing introns and reconnecting exons in a pre-
mRNA
template strand
the strand of DNA that specifies the complementary mRNA molecule
transcription bubble
the region of locally unwound DNA that allows for transcription of
mRNA
Translation
By the end of this section, you will be able to:
e Describe the different steps in protein synthesis
e Discuss the role of ribosomes in protein synthesis
e Describe the genetic code and how the nucleotide sequence determines
the amino acid and the protein sequence
The synthesis of proteins is one of a cell’s most energy-consuming
metabolic processes. In turn, proteins account for more mass than any other
component of living organisms (with the exception of water), and proteins
perform a wide variety of the functions of a cell. The process of translation,
or protein synthesis, involves decoding an mRNA message into a
polypeptide product. Amino acids are covalently strung together in lengths
ranging from approximately 50 amino acids to more than 1,000.
The Protein Synthesis Machinery
In addition to the mRNA template, many other molecules contribute to the
process of translation. The composition of each component may vary across
species; for instance, ribosomes may consist of different numbers of
ribosomal RNAs (rRNA) and polypeptides depending on the organism.
However, the general structures and functions of the protein synthesis
machinery are comparable from bacteria to human cells. Translation
requires the input of an mRNA template, ribosomes, tRNAs, and various
enzymatic factors ([link]).
Amino acids
Growing
protein chain Ribosome
tRNA
mRNA
The protein synthesis machinery
includes the large and small
subunits of the ribosome, mRNA,
and tRNA. (credit: modification of
work by NIGMS, NIH)
Ribosomes are located in the cytoplasm and endoplasmic reticulum of
eukaryotes. Ribosomes are made up of a large and a small subunit that
come together for translation. The small subunit is responsible for binding
the mRNA template, whereas the large subunit sequentially binds tRNAs, a
type of RNA molecule that brings amino acids to the growing chain of the
polypeptide. Each mRNA molecule is simultaneously translated by many
ribosomes, all synthesizing protein in the same direction.
Depending on the species, 40 to 60 types of tRNA exist in the cytoplasm.
Serving as adaptors, specific tRNAs bind to sequences on the mRNA
template and add the corresponding amino acid to the polypeptide chain.
Therefore, tRNAs are the molecules that actually “translate” the language
of RNA into the language of proteins. For each tRNA to function, it must
have its specific amino acid bonded to it. In the process of tRNA
“charging,” each tRNA molecule is bonded to its correct amino acid.
The Genetic Code
To summarize what we know to this point, the cellular process of
transcription generates messenger RNA (mRNA), a mobile molecular copy
of one or more genes with an alphabet of A, C, G, and uracil (U).
Translation of the mRNA template converts nucleotide-based genetic
information into a protein product. Protein sequences consist of 20
commonly occurring amino acids; therefore, it can be said that the protein
alphabet consists of 20 letters. Each amino acid is defined by a three-
nucleotide sequence called the triplet codon. The relationship between a
nucleotide codon and its corresponding amino acid is called the genetic
code.
Given the different numbers of “letters” in the mRNA and protein
“alphabets,” combinations of nucleotides corresponded to single amino
acids. Using a three-nucleotide code means that there are a total of 64 (4 x 4
x 4) possible combinations; therefore, a given amino acid is encoded by
more than one nucleotide triplet ({link]).
Second letter
uacl™ |ugc}ovs
UAA Stop|UGA Stop
UAG Stop|UGG Trp
First letter
®
$
=
E
=<
=
U
Cc
A
G
U
Cc
A
G
U
Cc
A
G
U
Cc
A
G
This figure shows the genetic code for
translating each nucleotide triplet, or
codon, in mRNA into an amino acid
or a termination signal in a nascent
protein. (credit: modification of work
by NIH)
Three of the 64 codons terminate protein synthesis and release the
polypeptide from the translation machinery. These triplets are called stop
codons. Another codon, AUG, also has a special function. In addition to
specifying the amino acid methionine, it also serves as the start codon to
initiate translation. The reading frame for translation is set by the AUG start
codon near the 5' end of the mRNA. The genetic code is universal. With a
few exceptions, virtually all species use the same genetic code for protein
synthesis, which is powerful evidence that all life on Earth shares a
common origin.
The Mechanism of Protein Synthesis
Just as with mRNA synthesis, protein synthesis can be divided into three
phases: initiation, elongation, and termination.
Protein synthesis begins with the formation of an initiation complex. This
complex involves the small ribosome subunit, the mRNA template,and a
tRNA that interacts with the AUG start codon, and is linked to the amino
acid methionine.
In polypeptide elongation the large ribosomal subunit consists of three
compartments: P, A, and E sites. The A site binds incoming charged tRNAs
(tRNAs with their attached specific amino acids). The P site binds charged
tRNAs carrying amino acids that have formed bonds with the growing
polypeptide chain but have not yet dissociated from their corresponding
tRNA. The E site releases dissociated tRNAs so they can be recharged with
free amino acids. The ribosome shifts one codon at a time, catalyzing each
process that occurs in the three sites. With each step, a charged tRNA enters
the complex, the polypeptide becomes one amino acid longer, and an
uncharged tRNA departs.
Ribosome large
subunit
Small Ribosomal
subunit
Polypeptide chain
Translation begins when a tRNA
anticodon recognizes a codon on
the mRNA. The large ribosomal
subunit joins the small subunit,
and a second tRNA is recruited.
As the mRNA moves relative to
the ribosome, the polypeptide
chain is formed. Entry of a release
factor into the A site terminates
translation and the components
dissociate.
Termination of translation occurs when a stop codon (UAA, UAG, or UGA)
is encountered. When the ribosome encounters the stop codon, the growing
polypeptide is released and the ribosome subunits dissociate and leave the
mRNA. After many ribosomes have completed translation, the mRNA is
degraded so the nucleotides can be reused in another transcription reaction.
Note:
Concept in Action
Etcta tll
Ha
openstax COLLEGE
Transcribe a gene and translate it to protein using complementary pairing
and the genetic code at this site.
Section Summary
The central dogma describes the flow of genetic information in the cell
from genes to mRNA to proteins. Genes are used to make MRNA by the
process of transcription; mRNA is used to synthesize proteins by the
process of translation. The genetic code is the correspondence between the
three-nucleotide mRNA codon and an amino acid. The genetic code is
“translated” by the tRNA molecules, which associate a specific codon with
a specific amino acid. The genetic code is degenerate because 64 triplet
codons in mRNA specify only 20 amino acids and three stop codons. This
means that more than one codon corresponds to an amino acid. Almost
every species on the planet uses the same genetic code.
The players in translation include the mRNA template, ribosomes, tRNAs,
and various enzymatic factors. The small ribosomal subunit binds to the
mRNA template. Translation begins at the initiating AUG on the mRNA.
The formation of bonds occurs between sequential amino acids specified by
the mRNA template according to the genetic code. The ribosome accepts
charged tRNAs, and as it steps along the mRNA, it catalyzes bonding
between the new amino acid and the end of the growing polypeptide. The
entire mRNA is translated in three-nucleotide “steps” of the ribosome.
When a stop codon is encountered, a release factor binds and dissociates the
components and frees the new protein.
Multiple Choice
Exercise:
Problem:
The RNA components of ribosomes are synthesized in the
a. cytoplasm
b. nucleus
c. nucleolus
d. endoplasmic reticulum
Solution:
C
Exercise:
Problem:
How long would the peptide be that is translated from this MRNA
sequence: 5'-AUGGGCUACCGA-3'?
ae ot
WN O&O
d.4
Solution:
D
Free Response
Exercise:
Problem:
Transcribe and translate the following DNA sequence (nontemplate
strand): 5'-ATGGCCGGTTATTAAGCA-3'
Solution:
The mRNA would be: 5'-AUGGCCGGUUAUUAAGCA-3'. The
protein would be: Met Ala Gly Tyr. Even though there are six codons,
the fifth codon corresponds to a stop (UAA) so the sixth codon would
not be translated.
Glossary
codon
three consecutive nucleotides in mRNA that specify the addition of a
specific amino acid or the release of a polypeptide chain during
translation
genetic code
the amino acids that correspond to three-nucleotide codons of MRNA
rRNA
ribosomal RNA; molecules of RNA that combine to form part of the
ribosome
stop codon
one of the three mRNA codons that specifies termination of translation
start codon
the AUG (or, rarely GUG) on an mRNA from which translation
begins; always specifies methionine
tRNA
transfer RNA; an RNA molecule that contains a specific three-
nucleotide anticodon sequence to pair with the mRNA codon and also
binds to a specific amino acid
Homeostasis
By the end of this section, you will be able to:
e Explain the concept of homeostasis
e Describe the general process of thermoregulation in humans
Before moving on to discussing organ systems, it is important to review the
concept of internal balance. Homeostasis refers to the relatively stable state
inside the body. Human organs and organ systems constantly adjust to
internal and external changes in order to maintain this steady state.
Examples of internal conditions maintained homeostatically are the level of
blood glucose, body temperature, and blood calcium level. These conditions
remain stable because of physiologic processes that result in negative
feedback relationships. If the blood glucose or calcium rises, this sends a
signal to organs responsible for lowering blood glucose or calcium. The
signals that restore the normal levels are examples of negative feedback.
When homeostatic mechanisms fail, the results can be unfavorable.
Homeostatic mechanisms keep the body in dynamic equilibrium by
constantly adjusting to the changes that the body’s systems encounter. Even
when a person is inactive, he/she is maintaining this homeostatic
equilibrium. An examples of a factor that is regulated homeostatically is
body temperature in a process called thermoregulation.
Homeostasis
The goal of homeostasis is the maintenance of equilibrium around a specific
value of some aspect of the body or its cells called a set point. While there
are normal fluctuations from the set point, the body’s systems will usually
attempt to go back to this point. A change in the internal or external
environment is called a stimulus and is detected by a receptor; the response
of the system is to adjust the activities of the system so the value moves
back toward the set point. For instance, if the body becomes too warm,
adjustments are made to cool it. If glucose levels in the blood rise after a
meal, adjustments are made to lower them and to get the nutrient into
tissues that need it or to store it for later use.
When a change occurs in an animal’s environment, an adjustment must be
made so that the internal environment of the body and cells remains stable.
The receptor that senses the change in the environment is part of a feedback
mechanism. The stimulus—temperature, glucose, or calcium levels—is
detected by the receptor. The receptor sends information to a control center,
often the brain, which relays appropriate signals to an effector organ that is
able to cause an appropriate change, either up or down, depending on the
information the sensor was sending.
Thermoregulation
Animals, such as humans, that maintain a constant body temperature in the
face of differing environmental temperatures, are called endotherms. We
are able to maintain this temperature by generating internal heat (a waste
product of the cellular chemical reactions of metabolism) that keeps the
cellular processes operating optimally even when the environment is cold.
Endotherms use their circulatory systems to help maintain body
temperature. Vasodilation, the opening up of arteries to the skin by
relaxation of their smooth muscles, brings more blood and heat to the body
surface, facilitating radiation and evaporative heat loss, cooling the body.
Vasoconstriction, the narrowing of blood vessels to the skin by contraction
of their smooth muscles, reduces blood flow in peripheral blood vessels,
forcing blood toward the core and vital organs, conserving heat.
Thermoregulation is coordinated by the nervous system ([link]). The
processes of temperature control are centered in a region of the brain called
the hypothalamus. The hypothalamus maintains the set point for body
temperature through reflexes that cause vasodilation or vasoconstriction and
shivering or sweating. The hypothalamus directs the responses that effect
the changes in temperature loss or gain that return the body to the set point.
The set point may be adjusted in some instances. During an infection,
compounds called pyrogens are produced and circulate to the hypothalamus
resetting the thermostat to a higher value. This allows the body’s
temperature to increase to a new homeostatic equilibrium point in what is
commonly called a fever. The increase in body heat makes the body less
optimal for bacterial growth and increases the activities of cells so they are
better able to fight the infection.
Note:
Art Connection
Body temperature falls. Body temperature rises.
N\A’ _\
Blood vessels constrict
so that heat is conserved.
Sweat glands do not
secrete fluid. Shivering
Blood vessels dilate,
resulting in heat loss to the
environment. Sweat glands
secrete fluid. As the fluid
evaporates, heat is lost
from the body.
Normal body
temperature
DP Heat is lost to
Heat is retained.
The body is able to regulate temperature in response to signals
from the nervous system.
(involuntary contraction of
muscles) generates heat,
which warms the body.
Section Summary
Homeostasis is a dynamic equilibrium that is maintained in body tissues
and organs. It is dynamic because it is constantly adjusting to the changes
that the systems encounter. It is an equilibrium because body functions are
kept within a normal range, with some fluctuations around a set point.
Art Connections
Exercise:
Problem:
[link] When bacteria are destroyed by leukocytes, pyrogens are
released into the blood. Pyrogens reset the body’s thermostat to a
higher temperature, resulting in fever. How might pyrogens cause the
body temperature to rise?
Solution:
[link] Pyrogens increase body temperature by causing the blood
vessels to constrict, inducing shivering, and stopping sweat glands
from secreting fluid.
Review Questions
Exercise:
Problem:
When faced with a sudden drop in environmental temperature, an
endothermic animal will
a. experience a drop in its body temperature
b. wait to see if it goes lower
c. increase muscle activity to generate heat
d. add fur or fat to increase insulation
Solution:
G
Exercise:
Problem: What is the cause of a fever of 38.3 °C (101 °F)?
a. too much heat produced by the body
b. upward adjustment of the body temperature set point
c. inadequate cooling mechanisms in the body
d. the heat caused by a viral or bacterial infection
Solution:
B
Free Response
Exercise:
Problem: Describe how the body’s mechanisms maintain homeostasis?
Solution:
The body has a sensor that detects a deviation of the state of the cells
or the body from the set point. The information is relayed to a control
center, usually the brain, where signals go to effectors. Those effectors
cause a negative feedback response that moves the state of the body in
a direction back toward the set point.
Glossary
ectotherm
an organism that relies primarily on environmental heat sources to
maintain its body temperature
endotherm
an organism that relies primarily on internal heat sources to maintain
its body temperature
interstitial fluid
the fluid found between cells in the body, similar in constitution to the
fluid component of blood, but without the high concentrations of
proteins
kidney
the organ that performs excretory and osmoregulatory functions
nephron
the functional unit of the kidney
osmoregulation
the mechanism by which water and solute concentrations are
maintained at desired levels
osmotic balance
the appropriate values of water and solute concentrations for a healthy
organism
renal artery
the artery that delivers blood to the kidney
renal vein
the vein that drains blood from the kidney
set point
the target value of a physiological state in homeostasis
ureter
the urine-bearing tubes coming out of the kidney
urethra
the tube that conducts urine from the urinary bladder to the external
environment
urinary bladder
the structure that the ureters empty the urine into
The Digestive System
By the end of this section, you will be able to:
e Explain the processes of digestion (chemical and physical) and
absorption
e Explain the specialized functions of the organs involved in processing
food in the body
e Describe the ways in which organs work together to digest food and
absorb nutrients
e List different digestive enzymes, their location of action, and the
nutrient that is their substrate
e Describe how excess carbohydrates and energy are stored in the body
All living organisms need nutrients to survive. While plants can obtain
nutrients from their roots and the energy molecules required for cellular
function through the process of photosynthesis, animals obtain their
nutrients by the consumption of other organisms. At the cellular level, the
biological molecules necessary for animal function are amino acids, lipid
molecules, nucleotides, and simple sugars. However, the food consumed
consists of protein, fat, and complex carbohydrates. Animals must convert
these macromolecules into the simple molecules required for maintaining
cellular function. The conversion of the food consumed to the nutrients
required is a multistep process involving digestion and absorption. During
digestion, food particles are broken down to smaller components, which are
later absorbed by the body. This happens by both physical means, such as
chewing, and by chemical means, via enzyme-catalyzed reactions.
One of the challenges in human nutrition is maintaining a balance between
food intake, storage, and energy expenditure. Taking in more food energy
than is used in activity leads to storage of the excess in the form of fat
deposits. The rise in obesity and the resulting diseases like type 2 diabetes
makes understanding the role of diet and nutrition in maintaining good
health all the more important.
The Human Digestive System
The process of digestion begins in the mouth (oral cavity) with the intake of
food ({link]). The teeth play an important role in masticating (chewing) or
physically breaking food into smaller particles. This action not only
decreases the size of the food particles to facilitate swallowing, but also
increases surface area for chemical digestion. The enzymes present in saliva
(amylase and lipase) also begin to chemically break down food (starch and
fats, respectively). The food is then swallowed and enters the esophagus—
a long tube that connects the mouth to the stomach. Using peristalsis, or
wave-like smooth-muscle contractions, the muscles of the esophagus push
the food toward the stomach. The stomach contents are extremely acidic,
with a pH between 1.5 and 2.5. This acidity kills microorganisms, breaks
down food tissues, and activates digestive enzymes. Further breakdown of
food takes place in the small intestine where bile produced by the liver, and
enzymes produced by the small intestine and the pancreas, continue the
process of digestion. The smaller molecules are absorbed into the blood
stream through the epithelial cells lining the walls of the small intestine.
The waste material travels on to the large intestine where water is absorbed
and the drier waste material is compacted into feces; it is stored in the
rectum until it is excreted through the anus.
Oral cavity
Esophagus
Liver Stomach
Gall bladder
Small intestine
Pancreas
Cecum
Large intestine
The components of the human
digestive system are shown. The
GI tract is the tube that includes the
oral cavity, esophagus, stomach,
small intestine, large intestine, and
rectum. The accessory organs are
those that indirectly join to this
tube via ducts and include the
salivary glands, liver, gall bladder,
and pancreas.
Oral Cavity
Both physical and chemical digestion begin in the mouth or oral cavity,
which is the point of entry of food into the digestive system. The food is
broken into smaller particles by mastication, the chewing action of the
teeth. All mammals have teeth and can chew their food to begin the process
of physically breaking it down into smaller particles.
The chemical process of digestion begins during chewing as food mixes
with saliva, produced by the salivary glands ((link]). Saliva contains
mucus that moistens food and buffers the pH of the food. Saliva also
contains lysozyme, which has antibacterial action. It also contains an
enzyme called salivary amylase that begins the process of converting
starches in the food into a disaccharide called maltose. Another enzyme
called lipase is produced by cells in the tongue to break down fats. The
chewing and wetting action provided by the teeth and saliva prepare the
food into a mass called the bolus for swallowing. The tongue helps in
swallowing—moving the bolus from the mouth into the pharynx. The
pharynx opens to two passageways: the esophagus and the trachea. The
esophagus leads to the stomach and the trachea leads to the lungs. The
epiglottis is a flap of tissue that covers the tracheal opening during
swallowing to prevent food from entering the lungs.
Nasal cavity
Salivary glands:
Parotid
Teeth Sublingual
Submandibular
(a) (b)
(a) Digestion of food begins in the mouth. (b) Food is
masticated by teeth and moistened by saliva secreted from the
salivary glands. Enzymes in the saliva begin to digest starches
and fats. With the help of the tongue, the resulting bolus is
moved into the esophagus by swallowing. (credit: modification
of work by Mariana Ruiz Villareal)
Esophagus
The esophagus is a tubular organ that connects the mouth to the stomach.
The chewed and softened food (i.e. the bolus) passes through the esophagus
after being swallowed. The smooth muscles of the esophagus undergo
peristalsis (contractions) that pushes the food toward the stomach. The
peristaltic wave is unidirectional—it moves food from the mouth the
stomach, and reverse movement is not possible, except in the case of the
vomit reflex. The peristaltic movement of the esophagus is an involuntary
reflex; it takes place in response to the act of swallowing and you don't
exert conscious control over it.
Ring-like muscles called sphincters form valves in the digestive system.
The gastro-esophageal sphincter (a.k.a. lower esophageal or cardiac
sphincter) is located at the stomach end of the esophagus. In response to
swallowing and the pressure exerted by the bolus of food, this sphincter
opens, and the bolus enters the stomach. When there is no swallowing
action, this sphincter is shut and prevents the contents of the stomach from
traveling up the esophagus. Acid reflux or “heartburn” occurs when the
acidic digestive juices escape back into the esophagus and the low pH
irritates the unprotected surface. Prolonged and repeated exposure of the
esophagus to this acidity can cause physical damage.
Stomach
A large part of protein digestion occurs in the stomach ({link]). The
stomach is a saclike organ that secretes gastric digestive juices.
Protein digestion is carried out by an enzyme called pepsin in the stomach
chamber. The highly acidic environment kills many microorganisms in the
food and, combined with the action of the enzyme pepsin, results in the
catabolism of protein in the food. Chemical digestion is facilitated by the
churning action of the stomach caused by contraction and relaxation of
smooth muscles. The partially digested food and gastric juice mixture is
called chyme. Gastric emptying occurs within two to six hours after a meal.
Only a small amount of chyme is released into the small intestine at a time.
The movement of chyme from the stomach into the small intestine is
regulated by hormones, stomach distension and muscular reflexes that
influence the pyloric sphincter. The low pH of the stomach will denature the
amylase and lipase that were secreted in the mouth. Therefore, over time,
chemical digestion of starches and fats will decrease in the stomach.
The stomach lining is unaffected by pepsin and the acidity because pepsin is
released in an inactive form (pepsinogen) that is activated by the low pH.
The stomach also has a thick mucus lining that protects the underlying
tissue.
Small Intestine
Chyme moves from the stomach to the small intestine. The small intestine
is the organ where the digestion of protein, fats, and carbohydrates is
completed. The small intestine is a long tube-like organ with a highly
folded surface containing finger-like projections called the villi. The top
surface of each villus has many microscopic projections called microvilli.
The epithelial cells at the surface of these structures absorb nutrients from
the digested food and release them to the bloodstream on the other side.
Methods of transport previously discussed (e.g.active transport)are used
during this movement. The villi and microvilli, with their many folds,
increase the surface area of the small intestine and increase absorption
efficiency of the nutrients.
The human small intestine is over 6 m (19.6 ft) long and is divided into
three parts: the duodenum, the jejunum and the ileum. The duodenum is
separated from the stomach by the pyloric sphincter. The chyme is mixed
with pancreatic juices, an alkaline/basic solution rich in bicarbonate that
neutralizes the acidity of chyme from the stomach. This result raises the pH
and creates an environment that is appropriate for enzymes. Pancreatic
juices contain several digestive enzymes (amylase, trypsin, and lipase) that
break down starches, proteins, and fats, respectively. Bile is produced in the
liver and stored and concentrated in the gallbladder; it enters the duodenum
through the bile duct. Bile contains bile salts, which make lipids accessible
to the water-soluble enzymes. This is accomplished via a process called
emulsification, a type of physical digestion. Bile keeps fat droplets from
coming back together again, thus increasing the surface area available to
lipase. The wall of the small intestines secrete disaccharidases, which
faciltate digestion of disaccharides (e.g. maltose, sucrose, and lactose) into
their respective monosaccharides. The monosaccharides, amino acids, bile
salts, vitamins, and other nutrients are absorbed by the cells of the intestinal
lining.
The undigested food is sent to the colon from the ileum via peristaltic
movements. The ileum ends and the large intestine begins at the ileocecal
valve. The vermiform, “worm-like,” appendix is located at the ileocecal
valve. The appendix of humans has a minor role in immunity.
Large Intestine
The large intestine reabsorbs the water from indigestible food material and
processes the waste material ([link]). The human large intestine is much
smaller in length compared to the small intestine but larger in diameter. It
has three parts: the cecum, the colon, and the rectum. The cecum joins the
ileum to the colon and is the receiving pouch for the waste matter. The
colon is home to many bacteria or “intestinal flora” that aid in the digestive
processes. The colon has four regions, the ascending colon, the transverse
colon, the descending colon and the sigmoid colon. The main functions of
the colon are to extract the water and mineral salts from undigested food,
and to store waste material.
Transverse
colon
Ascending
colon
Descending
colon
Cecum
Vermiform Sigmoid
appendix colon
Rectum
Anus
The large intestine reabsorbs water from
undigested food and stores waste until it is
eliminated. (credit: modification of work by
Mariana Ruiz Villareal)
The rectum ([link]) stores feces until defecation. The feces are propelled
using peristaltic movements during elimination. The anus is an opening at
the far-end of the digestive tract and is the exit point for the waste material.
Two sphincters regulate the exit of feces, the inner sphincter is involuntary
and the outer sphincter is voluntary.
Accessory Organs
The organs discussed above are the organs of the digestive tract through
which food passes. Accessory organs add secretions and enzymes that break
down food into nutrients. Accessory organs include the salivary glands, the
liver, the pancreas, and the gall bladder. The secretions of the liver,
pancreas, and gallbladder are regulated by hormones in response to food
consumption.
The liver is the largest internal organ in humans and it plays an important
role in digestion of fats and detoxifying blood. The liver produces bile, a
digestive juice that is required for the breakdown of fats in the duodenum.
The liver also processes the absorbed vitamins and fatty acids and
synthesizes many plasma proteins. The gallbladder is a small organ that
aids the liver by storing bile and concentrating bile salts.
The pancreas secretes bicarbonate that neutralizes the acidic chyme and a
variety of enzymes (trypsin, amylase, and lipase) for the digestion of
proteins, carbohydrates, and fats, respectively.
Note:
Art Connection
Liver
Stomach
Gallbladder
Pancreas
Colon
Transverse colon Small Intestine
A di | Duodenum
scending colon Jejunum
Descending colon ta - BTA” Ue lleum
Cecum
Appendix
The stomach has an extremely acidic environment where most of
the protein gets digested. (credit: modification of work by Mariana
Ruiz Villareal)
Nutrition
The human diet should be well balanced to provide nutrients required for
bodily function and the minerals and vitamins required for maintaining
structure and regulation necessary for good health and reproductive
capability ({link]).
Vegetables ry
Choose OV
For humans, a balanced diet
includes fruits, vegetables,
grains, protein, and dairy.
(credit: USDA)
Note:
Concept in Action
.
— .
meee, OPENStAX COLLEGE
ae
(a) Ne.
Explore this interactive United States Department of Agriculture website to
learn more about each food group and the recommended daily amounts.
The organic molecules required for building cellular material and tissues
must come from food. During digestion, digestible carbohydrates are
ultimately broken down into glucose and used to provide energy within the
cells of the body. Complex carbohydrates, including polysaccharides, can
be broken down into glucose through biochemical modification; however,
humans do not produce the enzyme necessary to digest cellulose (fiber).
The intestinal flora in the human gut are able to extract some nutrition from
these plant fibers. These plant fibers are known as dietary fiber and are an
important component of the diet. The excess sugars in the body are
converted into glycogen and stored for later use in the liver and muscle
tissue. Glycogen stores are used to fuel prolonged exertions, such as long-
distance running, and to provide energy during food shortage. Fats are
stored under the skin of mammals for insulation and energy reserves.
Proteins in food are broken down during digestion and the resulting amino
acids are absorbed. All of the proteins in the body must be formed from
these amino-acid constituents; no proteins are obtained directly from food.
Fats add flavor to food and promote a sense of satiety or fullness. Fatty
foods are also significant sources of energy, and fatty acids are required for
the construction of lipid membranes. Fats are also required in the diet to aid
the absorption of fat-soluble vitamins and the production of fat-soluble
hormones.
While the animal body can synthesize many of the molecules required for
function from precursors, there are some nutrients that must be obtained
from food. These nutrients are termed essential nutrients, meaning they
must be eaten, because the body cannot produce them. Essential nutrients
include some fatty acids, some amino acids, vitamins, and minerals.
Section Summary
There are many organs that work together to digest food and absorb
nutrients. The mouth is the point of ingestion and the location where both
mechanical and chemical breakdown of food begins. Saliva contains an
enzyme called amylase that breaks down carbohydrates and an enxyme
lipase that breaks down triglycerides. The food bolus travels through the
esophagus by peristaltic movements to the stomach. The stomach has an
extremely acidic environment. The enzyme pepsin digests protein in the
stomach. Further digestion and absorption take place in the small intestine.
The large intestine reabsorbs water from the undigested food and stores
waste until elimination.
Carbohydrates, proteins, and fats are the primary components of food.
Some essential nutrients are required for cellular function but cannot be
produced by the animal body. These include vitamins (both fat and water
soluble) , minerals, some fatty acids, and some amino acids. Food intake in
more than necessary amounts is stored as glycogen in the liver and muscle
cells, and in adipose tissue. Excess adipose storage can lead to obesity and
serious health problems.
Art Connections
Exercise:
Problem:
[link] Which of the following statements about the digestive system is
false?
a. Chyme is a mixture of food and digestive juices that is produced
in the stomach.
b. Food enters the large intestine before the small intestine.
c. In the small intestine, chyme mixes with bile, which emulsifies
fats.
d. The stomach is separated from the small intestine by the pyloric
sphincter.
Solution:
[link] B
Review Questions
Exercise:
Problem: Where does the majority of fat digestion take place?
a. mouth
b. stomach
c. small intestine
d. large intestine
Solution:
C
Exercise:
Problem:The bile from the liver is delivered to the
a. stomach
b. liver
c. small intestine
d. colon
Solution:
C
Exercise:
Problem: Which of the following statements is not true?
a. Essential nutrients can be synthesized by the body.
b. Vitamins are required in small quantities for bodily function.
c. Some amino acids can be synthesized by the body, while others
need to be obtained from diet.
d. Vitamins come in two categories: fat-soluble and water-soluble.
Solution:
A
Free Response
Exercise:
Problem: What is the role of the accessory organs in digestion?
Solution:
Accessory organs play an important role in producing and delivering
digestive juices to the intestine during digestion and absorption.
Specifically, the salivary glands, liver, pancreas, and gallbladder play
important roles. Malfunction of any of these organs can lead to disease
states.
Glossary
amylase
an enzyme found in saliva and secreted by the pancreas that converts
carbohydrates to maltose
anus
the exit point of the digestive system for waste material
bile
a digestive juice produced by the liver; important for digestion of
lipids
bolus
a mass of food resulting from chewing action and wetting by saliva
colon
the largest portion of the large intestine consisting of the ascending
colon, transverse colon, and descending colon
chyme
a mixture of partially digested food and stomach juices
esophagus
a tubular organ that connects the mouth to the stomach
essential nutrient
a nutrient that cannot be synthesized by the body; it must be obtained
from food
gallbladder
the organ that stores and concentrates bile
large intestine
a digestive system organ that reabsorbs water from undigested material
and processes waste matter
liver
an organ that produces bile for digestion and processes vitamins and
lipids
mineral
an inorganic, elemental molecule that carries out important roles in the
body
oral cavity
the point of entry of food into the digestive system
pancreas
a gland that secretes digestive juices
pepsin
an enzyme found in the stomach whose main role is protein digestion
peristalsis
wave-like movements of muscle tissue
rectum
the area of the body where feces is stored until elimination
salivary gland
one of three pairs of exocrine glands in the mammalian mouth that
secretes saliva, a mix of watery mucus and enzymes
small intestine
the organ where digestion of protein, fats, and carbohydrates is
completed
stomach
a saclike organ containing acidic digestive juices
vitamin
an organic substance necessary in small amounts to sustain life
Introduction to Metabolism
class="introduction"
A
hummingbir
d needs
energy to
maintain
prolonged
flight. The
bird obtains
its energy
from taking
in food and
transforming
the energy
contained in
food
molecules
into forms of
energy to
power its
flight
through a
series of
biochemical
reactions.
(credit:
modification
of work by
Cory
Zanker)
Virtually every task performed by living organisms requires energy. Energy
is needed to perform heavy labor and exercise, but humans also use energy
while thinking, and even during sleep. In fact, the living cells of every
organism constantly use energy. Nutrients and other molecules are imported
into the cell, metabolized (broken down) and possibly synthesized into new
molecules, modified if needed, transported around the cell, and possibly
distributed to the entire organism. For example, the large proteins that make
up muscles are built from smaller molecules imported from dietary amino
acids. Complex carbohydrates are broken down into simple sugars that the
cell uses for energy. Just as energy is required to both build and demolish a
building, energy is required for the synthesis and breakdown of molecules
as well as the transport of molecules into and out of cells. In addition,
processes such as ingesting and breaking down pathogenic bacteria and
viruses, exporting wastes and toxins, and movement of the cell require
energy. From where, and in what form, does this energy come? How do
living cells obtain energy, and how do they use it? This chapter will discuss
different forms of energy and the physical laws that govern energy transfer.
This chapter will also describe how cells use energy and replenish it, and
how chemical reactions in the cell are performed with great efficiency.
Energy and Metabolism
By the end of this section, you will be able to:
e Explain what metabolic pathways are
e State the first and second laws of thermodynamics
e Explain the difference between kinetic and potential energy
e Describe endergonic and exergonic reactions
e Discuss how enzymes function as molecular catalysts
Metabolic Pathways
Cellular processes such as the building and breaking down of complex
molecules occur through stepwise chemical reactions. Some of these
chemical reactions are spontaneous and release energy, whereas others
require energy to proceed. Just as living things must continually consume
food to replenish their energy supplies, cells must continually produce more
energy to replenish that used by the many energy-requiring chemical
reactions that constantly take place. Together, all of the chemical reactions
that take place inside cells, including those that consume or generate
energy, are referred to as the cell’s metabolism. Consider the metabolism of
sugar. This is a classic example of one of the many cellular processes that
use and produce energy. Living things consume sugars as a major energy
source, because sugar molecules have a great deal of energy stored within
their bonds. For the most part, photosynthesizing organisms like plants
produce these sugars. During photosynthesis, plants use energy (originally
from sunlight) to convert carbon dioxide gas (CO>) into sugar molecules
(like glucose: CgH; 0¢). They consume carbon dioxide and produce
oxygen as a waste product. This reaction is summarized as:
Equation:
6CO, + 6H2O --> CgH}20¢ + 602
Because this process involves synthesizing an energy-storing molecule, it
requires energy input to proceed. During the light reactions of
photosynthesis, energy is provided by a molecule called adenosine
triphosphate (ATP), which is the primary energy currency of all cells. Just
as the dollar is used as currency to buy goods, cells use molecules of ATP
as energy currency to perform immediate work. In contrast, energy-storage
molecules such as glucose are consumed only to be broken down to use
their energy. The reaction that harvests the energy of a sugar molecule in
cells requiring oxygen to survive can be summarized by the reverse reaction
to photosynthesis. In this reaction, oxygen is consumed and carbon dioxide
is released as a waste product. The reaction is summarized as:
Equation:
C.gH120¢ + 60, --> 6H,O + 6CO,
Both of these reactions involve many steps.
The processes of making and breaking down sugar molecules illustrate two
examples of metabolic pathways. A metabolic pathway is a series of
chemical reactions that takes a starting molecule and modifies it, step-by-
step, through a series of metabolic intermediates, eventually yielding a final
product. In the example of sugar metabolism, the first metabolic pathway
synthesized sugar from smaller molecules, and the other pathway broke
sugar down into smaller molecules. These two opposite processes—the first
requiring energy and the second producing energy—are referred to as
anabolic pathways (building polymers) and catabolic pathways (breaking
down polymers into their monomers), respectively. Consequently,
metabolism is composed of synthesis (anabolism) and degradation
(catabolism) ({link]).
It is important to know that the chemical reactions of metabolic pathways
do not take place on their own. Each reaction step is facilitated, or
catalyzed, by a protein called an enzyme. Enzymes are important for
catalyzing all types of biological reactions—those that require energy as
well as those that release energy.
Metabolic pathways
Anabolic: Small molecules are built into large ones. Energy is required.
oeeo : = 0000
Catabolic: Large molecules are broken down into small ones. Energy is released.
©0002 — © © © O
Catabolic pathways are those that generate energy by breaking
down larger molecules. Anabolic pathways are those that
require energy to synthesize larger molecules. Both types of
pathways are required for maintaining the cell’s energy
balance.
Energy
Thermodynamics refers to the study of energy and energy transfer
involving physical matter. The matter relevant to a particular case of energy
transfer is called a system, and everything outside of that matter is called
the surroundings. For instance, when heating a pot of water on the stove,
the system includes the stove, the pot, and the water. Energy is transferred
within the system (between the stove, pot, and water). There are two types
of systems: open and closed. In an open system, energy can be exchanged
with its surroundings. The stovetop system is open because heat can be lost
to the air. A closed system cannot exchange energy with its surroundings.
Biological organisms are open systems. Energy is exchanged between them
and their surroundings as they use energy from the sun to perform
photosynthesis or consume energy-storing molecules and release energy to
the environment by doing work and releasing heat. Like all things in the
physical world, energy is subject to physical laws. The laws of
thermodynamics govern the transfer of energy in and among all systems in
the universe.
In general, energy is defined as the ability to do work, or to create some
kind of change. Energy exists in different forms. For example, electrical
energy, light energy, and heat energy are all different types of energy. To
appreciate the way energy flows into and out of biological systems, it is
important to understand two of the physical laws that govern energy.
Thermodynamics
The first law of thermodynamics states that the total amount of energy in
the universe is constant and conserved. In other words, there has always
been, and always will be, exactly the same amount of energy in the
universe. Energy exists in many different forms. According to the first law
of thermodynamics, energy may be transferred from place to place or
transformed into different forms, but it cannot be created or destroyed. The
transfers and transformations of energy take place around us all the time.
Light bulbs transform electrical energy into light and heat energy. Gas
stoves transform chemical energy from natural gas into heat energy. Plants
perform one of the most biologically useful energy transformations on
earth: that of converting the energy of sunlight to chemical energy stored
within organic molecules. Some examples of energy transformations are
shown in [link].
The challenge for all living organisms is to obtain energy from their
surroundings in forms that they can transfer or transform into usable energy
to do work. Living cells have evolved to meet this challenge. Chemical
energy stored within organic molecules such as sugars and fats is
transferred and transformed through a series of cellular chemical reactions
into energy within molecules of ATP. Energy in ATP molecules is easily
accessible to do work. Examples of the types of work that cells need to do
include building complex molecules, transporting materials, powering the
motion of cilia or flagella, and contracting muscle fibers to create
movement.
Chemical energy Light energy
> >
Kinetic energy Chemical energy
*
Shown are some examples of energy
transferred and transformed from one
system to another and from one form to
another. The food we consume provides our
cells with the energy required to carry out
bodily functions, just as light energy
provides plants with the means to create the
chemical energy they need. (credit "ice
cream": modification of work by D. Sharon
Pruitt; credit "kids": modification of work
by Max from Providence; credit "leaf":
modification of work by Cory Zanker)
A living cell’s primary tasks of obtaining, transforming, and using energy to
do work may seem simple. However, the second law of thermodynamics
explains why these tasks are harder than they appear. The second law of
thermodynamics says that energy will always be lost as heat in energy
transfers or transformations. All energy transfers and transformations are
never completely efficient. In every energy transfer, some amount of energy
is lost in a form that is unusable. In most cases, this form is heat energy.
Thermodynamically, heat energy is defined as the energy transferred from
one system to another that is not work. For example, when a light bulb is
turned on, some of the energy being converted from electrical energy into
light energy is lost as heat energy. Likewise, some energy is lost as heat
energy during cellular metabolic reactions.
Potential and Kinetic Energy
When an object is in motion, there is energy associated with that object.
Think of a wrecking ball. Even a slow-moving wrecking ball can do a great
deal of damage to other objects. Energy associated with objects in motion is
called kinetic energy ((link]). A speeding bullet, a walking person, and the
rapid movement of molecules in the air (which produces heat) all have
kinetic energy.
Now what if that same motionless wrecking ball is lifted two stories above
ground with a crane? If the suspended wrecking ball is unmoving, is there
energy associated with it? The answer is yes. The energy that was required
to lift the wrecking ball did not disappear, but is now stored in the wrecking
ball by virtue of its position and the force of gravity acting on it. This type
of energy is called potential energy ((link]). If the ball were to fall, the
potential energy would be transformed into kinetic energy until all of the
potential energy was exhausted when the ball rested on the ground.
Wrecking balls also swing like a pendulum; through the swing, there is a
constant change of potential energy (highest at the top of the swing) to
kinetic energy (highest at the bottom of the swing). Other examples of
potential energy include the energy of water held behind a dam or a person
about to skydive out of an airplane.
Still water has potential energy; moving water, such as in
a waterfall or a rapidly flowing river, has kinetic energy.
(credit "dam": modification of work by "Pascal"/Flickr;
credit "waterfall": modification of work by Frank
Gualtieri)
Potential energy is not only associated with the location of matter, but also
with the structure of matter. Even a spring on the ground has potential
energy if it is compressed; so does a rubber band that is pulled taut. On a
molecular level, the bonds that hold the atoms of molecules together exist in
a particular structure that has potential energy. Remember that anabolic
cellular pathways require energy to synthesize complex molecules from
simpler ones and catabolic pathways release energy when complex
molecules are broken down. The fact that energy can be released by the
breakdown of certain chemical bonds implies that those bonds have
potential energy. In fact, there is potential energy stored within the bonds of
all the food molecules we eat, which is eventually harnessed for use. This is
because these bonds can release energy when broken. The type of potential
energy that exists within chemical bonds, and is released when those bonds
are broken, is called chemical energy. Chemical energy is responsible for
providing living cells with energy from food. The release of energy occurs
when the molecular bonds within food molecules are broken.
Free and Activation Energy
After learning that chemical reactions release energy when energy-storing
bonds are broken, an important next question is the following: How is the
energy associated with these chemical reactions quantified and expressed?
How can the energy released from one reaction be compared to that of
another reaction? A measurement of free energy is used to quantify these
energy transfers. Recall that according to the second law of
thermodynamics, all energy transfers involve the loss of some amount of
energy in an unusable form such as heat. Free energy specifically refers to
the energy associated with a chemical reaction that is available after the
losses are accounted for. In other words, free energy is usable energy, or
energy that is available to do work.
If energy is released during a chemical reaction, then the change in free
energy, signified as AG (delta G) will be a negative number. A negative
change in free energy also means that the products of the reaction have less
free energy than the reactants, because they release some free energy during
the reaction. Reactions that have a negative change in free energy and
consequently release free energy are called exergonic reactions. Think:
exergonic means energy is exiting the system. These reactions are also
referred to as spontaneous reactions, and their products have less stored
energy than the reactants. An important distinction must be drawn between
the term spontaneous and the idea of a chemical reaction occurring
immediately. Contrary to the everyday use of the term, a spontaneous
reaction is not one that suddenly or quickly occurs. The rusting of iron is an
example of a spontaneous reaction that occurs slowly, little by little, over
time.
If a chemical reaction absorbs energy rather than releases energy on
balance, then the AG for that reaction will be a positive value. In this case,
the products have more free energy than the reactants. Thus, the products of
these reactions can be thought of as energy-storing molecules. These
chemical reactions are called endergonic reactions and they are non-
spontaneous. An endergonic reaction will not take place on its own without
the addition of free energy.
There is another important concept that must be considered regarding
endergonic and exergonic reactions. Exergonic reactions require a small
amount of energy input to get going, before they can proceed with their
energy-releasing steps. These reactions have a net release of energy, but still
require some energy input in the beginning. This small amount of energy
input necessary for all chemical reactions to occur is called the activation
energy.
Enzymes
A substance that helps a chemical reaction to occur is called a catalyst, and
the molecules that catalyze biochemical reactions are called enzymes. Most
enzymes are proteins and perform the critical task of lowering the activation
energies of chemical reactions inside the cell. Most of the reactions critical
to a living cell happen too slowly at normal temperatures to be of any use to
the cell. Without enzymes to speed up these reactions, life could not persist.
Enzymes do this by binding to the reactant molecules and holding them in
such a way as to make the chemical bond-breaking and -forming processes
take place more easily. It is important to remember that enzymes do not
change whether a reaction is exergonic (spontaneous) or endergonic. This is
because they do not change the free energy of the reactants or products.
They only reduce the activation energy required for the reaction to go
forward ([link]). In addition, an enzyme itself is unchanged by the reaction
it catalyzes. Once one reaction has been catalyzed, the enzyme is able to
participate in other reactions.
Activation
energy
reactants
Reaction path
Enzymes lower the activation energy
of the reaction but do not change the
free energy of the reaction.
The chemical reactants to which an enzyme binds are called the enzyme’s
substrates. There may be one or more substrates, depending on the
particular chemical reaction. In some reactions, a single reactant substrate is
broken down into multiple products. In others, two substrates may come
together to create one larger molecule. Two reactants might also enter a
reaction and both become modified, but they leave the reaction as two
products. The location within the enzyme where the substrate binds is
called the enzyme’s active site. The active site is where the “action”
happens. Since enzymes are proteins, there is a unique combination of
amino acid side chains within the active site. Each side chain is
characterized by different properties. They can be large or small, weakly
acidic or basic, hydrophilic or hydrophobic, positively or negatively
charged, or neutral. The unique combination of side chains creates a very
specific chemical environment within the active site. This specific
environment is suited to bind to one specific chemical substrate (or
substrates).
Active sites are subject to influences of the local environment. Increasing
the environmental temperature generally increases reaction rates, enzyme-
catalyzed or otherwise. However, temperatures outside of an optimal range
reduce the rate at which an enzyme catalyzes a reaction. Hot temperatures
will eventually cause enzymes to denature, an irreversible change in the
three-dimensional shape and therefore the function of the enzyme. Enzymes
are also suited to function best within a certain pH and salt concentration
range, and, as with temperature, extreme pH, and salt concentrations can
cause enzymes to denature.
Enzyme changes shape Products
Substrate slightly as substrate binds
[ ra site 5 /
Substrate entering Enzyme/substrate Enzyme/product Products leaving
the active site of complex complex the active site of
the enzyme the enzyme
In this diagram, a substrate binds the active site of
an enzyme and, in the process, both the shape of
the enzyme and the shape of the substrate change.
The substrate is converted to product, which leaves
the active site.
Enzymes can also be regulated in ways that either promote or reduce
enzyme activity. There are many kinds of molecules that inhibit or promote
enzyme function, and various mechanisms by which they do so. In some
cases of enzyme inhibition, an inhibitor molecule is similar enough to a
substrate that it can bind to the active site and simply block the substrate
from binding. When this happens, the enzyme is inhibited through
competitive inhibition, because an inhibitor molecule competes with the
substrate for binding to the active site.
Section Summary
Cells perform the functions of life through various chemical reactions. A
cell’s metabolism refers to the combination of chemical reactions that take
place within it. Catabolic reactions break down complex chemicals into
simpler ones and are associated with energy release. Anabolic processes
build complex molecules out of simpler ones and require energy.
In studying energy, the term system refers to the matter and environment
involved in energy transfers. Entropy is a measure of the disorder of a
system. The physical laws that describe the transfer of energy are the laws
of thermodynamics. The first law states that the total amount of energy in
the universe is constant. The second law of thermodynamics states that
every energy transfer involves some loss of energy in an unusable form,
such as heat energy. Energy comes in different forms: kinetic, potential, and
free. The change in free energy of a reaction can be negative (releases
energy, exergonic) or positive (consumes energy, endergonic). All reactions
require an initial input of energy to proceed, called the activation energy.
Enzymes are chemical catalysts that speed up chemical reactions by
lowering their activation energy. Enzymes have an active site with a unique
chemical environment that fits particular chemical reactants for that
enzyme, called substrates. Enzyme action is regulated to conserve resources
and respond optimally to the environment.
Review Questions
Exercise:
Problem:
Which of the following is not an example of an energy transformation?
a. Heating up dinner in a microwave
b. Solar panels at work
c. Formation of static electricity
d. None of the above
Solution:
D
Exercise:
Problem: Which of the following is not true about enzymes?
a. They are consumed by the reactions they catalyze.
b. They are usually made of amino acids.
c. They lower the activation energy of chemical reactions.
d. Each one is specific to the particular substrate(s) to which it
binds.
Solution:
A
Free Response
Exercise:
Problem:
Does physical exercise to increase muscle mass involve anabolic
and/or catabolic processes? Give evidence for your answer.
Solution:
Physical exercise involves both anabolic and catabolic processes. Body
cells break down sugars to provide ATP to do the work necessary for
exercise, such as muscle contractions. This is catabolism. Muscle cells
also must repair muscle tissue damaged by exercise by building new
muscle. This is anabolism.
Glossary
activation energy
the amount of initial energy necessary for reactions to occur
active site
a specific region on the enzyme where the substrate binds
allosteric inhibition
the mechanism for inhibiting enzyme action in which a regulatory
molecule binds to a second site (not the active site) and initiates a
conformation change in the active site, preventing binding with the
substrate
anabolic
describes the pathway that requires a net energy input to synthesize
complex molecules from simpler ones
bioenergetics
the concept of energy flow through living systems
catabolic
describes the pathway in which complex molecules are broken down
into simpler ones, yielding energy as an additional product of the
reaction
competitive inhibition
a general mechanism of enzyme activity regulation in which a
molecule other than the enzyme’s substrate is able to bind the active
site and prevent the substrate itself from binding, thus inhibiting the
overall rate of reaction for the enzyme
endergonic
describes a chemical reaction that results in products that store more
chemical potential energy than the reactants
enzyme
a molecule that catalyzes a biochemical reaction
exergonic
describes a chemical reaction that results in products with less
chemical potential energy than the reactants, plus the release of free
energy
feedback inhibition
a mechanism of enzyme activity regulation in which the product of a
reaction or the final product of a series of sequential reactions inhibits
an enzyme for an earlier step in the reaction series
heat energy
the energy transferred from one system to another that is not work
kinetic energy
the type of energy associated with objects in motion
metabolism
all the chemical reactions that take place inside cells, including those
that use energy and those that release energy
noncompetitive inhibition
a general mechanism of enzyme activity regulation in which a
regulatory molecule binds to a site other than the active site and
prevents the active site from binding the substrate; thus, the inhibitor
molecule does not compete with the substrate for the active site;
allosteric inhibition is a form of noncompetitive inhibition
potential energy
the type of energy that refers to the potential to do work
substrate
a molecule on which the enzyme acts
thermodynamics
the science of the relationships between heat, energy, and work
Glycolysis
By the end of this section, you will be able to:
e Explain how ATP is used by the cell as an energy source
e Describe the overall result in terms of molecules produced of the
breakdown of glucose by glycolysis
Even exergonic, energy-releasing reactions require a small amount of
activation energy to proceed. However, consider endergonic reactions,
which require much more energy input because their products have more
free energy than their reactants. Within the cell, where does energy to power
such reactions come from? The answer lies with an energy-supplying
molecule called adenosine triphosphate, or ATP. ATP is a small, relatively
simple molecule, but within its bonds contains the potential for a quick
burst of energy that can be harnessed to perform cellular work. This
molecule can be thought of as the primary energy currency of cells in the
same way that money is the currency that people exchange for things they
need. ATP is used to power the majority of energy-requiring cellular
reactions.
ATP in Living Systems
A living cell cannot store significant amounts of free energy. Excess free
energy would result in an increase of heat in the cell, which would denature
enzymes and other proteins, and thus destroy the cell. Rather, a cell must be
able to store energy safely and release it for use only as needed. Living cells
accomplish this using ATP, which can be used to fill any energy need of the
cell. How? It functions as a rechargeable battery.
When ATP is broken down, usually by the removal of its terminal
phosphate group, energy is released. This energy is used to do work by the
cell, usually by the binding of the released phosphate to another molecule,
thus activating it. For example, in the mechanical work of muscle
contraction, ATP supplies energy to move the contractile muscle proteins.
ATP Structure and Function
At the heart of ATP is a molecule of adenosine monophosphate (AMP),
which is composed of an adenine molecule bonded to both a ribose
molecule and a single phosphate group ([link]). Ribose is a five-carbon
sugar found in RNA and AMP is one of the nucleotides in RNA. The
addition of a second phosphate group to this core molecule results in
adenosine diphosphate (ADP); the addition of a third phosphate group
forms adenosine triphosphate (ATP).
Gamma Alpha =
phosphate phosphate —N
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Beta
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group
Ribose
The structure of ATP shows the basic
components of a two-ring adenine, five-
carbon ribose, and three phosphate
groups.
The addition of a phosphate group to a molecule requires a high amount of
energy and results in a high-energy bond. Phosphate groups are negatively
charged and thus repel one another when they are arranged in series, as they
are in ADP and ATP. This repulsion makes the ADP and ATP molecules
inherently unstable. The release of one or two phosphate groups from ATP,
a process called hydrolysis, releases energy. Hydrolysis occurs when water
is added to break the chemical bond and is the opposite of dehydration
synthesis reactions discussed previously.
Glycolysis
You have read that nearly all of the energy used by living things comes to
them in the bonds of the sugar, glucose. Glycolysis is the first step in the
breakdown of glucose to extract energy for cell metabolism. Many living
organisms carry out glycolysis as part of their metabolism. Glycolysis takes
place in the cytoplasm of eukaryotic cells.
Glycolysis begins with the six-carbon, ring-shaped structure of a single
glucose molecule and ends with two molecules of a three-carbon sugar
called pyruvate. Glycolysis consists of two distinct phases. In the first part
of the glycolysis pathway, energy is used to make adjustments so that the
six-carbon sugar molecule can be split evenly into two three-carbon
pyruvate molecules. In the second part of glycolysis, ATP and
nicotinamide-adenine dinucleotide (NADH) are produced ({link]). NAD+ is
the form of the coenzyme that is able to accept electrons and hydrogen from
the glucose. NADH carries the electrons to a later stage in metabolism to be
used to provide energy (indirectly) to catalyze the endergonic reaction of
adding a phosphate group to ADP to make ATP.
If the cell cannot catabolize the pyruvate molecules further, it will harvest
only two ATP molecules from one molecule of glucose. For example,
mature mammalian red blood cells are only capable of glycolysis, which is
their sole source of ATP. If glycolysis is interrupted, these cells would
eventually die.
es Glucose
2 ATP
2 ADP
Fructose
diphosphate
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CCCOm—ECe CO
Glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate
NAD* NAD*
NADH NADH
2 ADP 2 ADP
2 ATP 2 ATP
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Pyruvate Pyruvate
In glycolysis, a glucose
molecule is converted into two
pyruvate molecules. Notice that
each of the two molecules has
three carbon atoms,
representing the six carbons that
were present in the glucose that
started the process. This means
that no carbon dioxide is
released in glycolysis, as the
carbon to make it comes from
the glucose molecule.
Section Summary
ATP functions as the energy currency for cells. It allows cells to store
energy briefly and transport it within itself to support endergonic chemical
reactions. The structure of ATP is that of an RNA nucleotide with three
phosphate groups attached. As ATP is used for energy, a phosphate group is
detached, and ADP is produced. Energy derived from glucose catabolism is
used to recharge ADP into ATP.
Glycolysis is the first pathway used in the breakdown of glucose to extract
energy. Because it is used by nearly all organisms on earth, it must have
evolved early in the history of life. Glycolysis consists of two parts: The
first part prepares the six-carbon ring of glucose for separation into two
three-carbon sugars. Energy from ATP is invested into the molecule during
this step to energize the separation. The second half of glycolysis extracts
ATP and high-energy electrons from hydrogen atoms and attaches them to
NAD". Two ATP molecules are invested in the first half and four ATP
molecules are formed during the second half. This produces a net gain of
two ATP molecules per molecule of glucose for the cell.
Multiple Choice
Exercise:
Problem:
Energy is stored long-term in the bonds of and used short-term
to perform work from a(n) molecule.
a. ATP : glucose
b. an anabolic molecule : catabolic molecule
c. glucose : ATP
d. a catabolic molecule : anabolic molecule
Solution:
G
Exercise:
Problem:The energy currency used by cells is
a. ATP
b. ADP
c. AMP
d. adenosine
Solution:
A
Exercise:
Problem:
The glucose that enters the glycolysis pathway is split into two
molecules of
a. ATP
b. phosphate
c. NADH
d. pyruvate
Solution:
D
Glossary
ATP
(also, adenosine triphosphate) the cell’s energy currency
glycolysis
the process of breaking glucose into two three-carbon molecules with
the production of ATP and NADH
The Transition Reaction, Citric Acid/Kreb's Cycle and Electron Transport
Chain/Oxidative Phosphorylation
By the end of this section, you will be able to:
e State the location of these reactions in the cell
e Describe the overall outcome of the transition reaction, citric
acid/Kreb's cycle and the electron transport chain/oxidative
phosphorylation in terms of the products of each
e Describe the relationships of glycolysis, transition reaction, citric
acid/Kreb's cycle, and electron transport chain/oxidative
phosphorylation in terms of their inputs and outputs.
The Transition Reaction and Citric Acid/Kreb's Cycle
In eukaryotic cells, the pyruvate molecules produced at the end of
glycolysis are transported into mitochondria, which are sites of cellular
respiration. If oxygen is available, aerobic respiration will go forward. In
mitochondria, pyruvate will be transformed into a two-carbon acetyl group
(by removing a molecule of carbon dioxide) that will be picked up by a
carrier compound called coenzyme A (CoA), which is made from vitamin
B. The resulting compound is called acetyl CoA. ((link]). This set of
reactions is referred to as the transition reaction, as it happens during
pyruvate transport into the mitochondria. The major function of acetyl CoA
is to deliver the acetyl group (2 carbon fragment) derived from pyruvate to
the next pathway in glucose catabolism, which is the citric acid/Kreb's
cycle. Note that during the transition reaction, each pyruvate/pyruvic acid
molecule loses one carbon as carbon dioxide and one molecule of NADH is
produced. Therefore, a total of two molecules of carbon dioxide and two
molecules of NADH are produced per glucose that started glycolysis.
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Pyruvic acid Acetyl CoA
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Citric
acid
cycle
During the transition reaction, pyruvate is
converted into acetyl-CoA before entering the
citric acid/Kreb's cycle.
Like the conversion of pyruvate to acetyl CoA, the citric acid cycle (also
called the Kreb's cycle) in eukaryotic cells takes place in the matrix of the
mitochondria. Unlike glycolysis, the citric acid cycle is a closed loop: The
last part of the pathway regenerates the compound used in the first step. The
eight steps of the cycle are a series of chemical reactions that produces two
carbon dioxide molecules, one ATP molecule (or an equivalent), and
reduced forms (NADH and FADH>) of NAD* and FAD", important
coenzymes in the cell. Part of this is considered an aerobic pathway
(oxygen-requiring) because the NADH and FADH) produced must transfer
their electrons to the next pathway in the system, which will use oxygen. If
oxygen is not present, this transfer does not occur. Note that per glucose
that started glycolysis, processing of the two pyruvate/pyruvic acid
molecules in the citric acid cycle will result in the production of a total of
six NADH, two FADHb, and two ATP. Also note that at this point, a total of
six molecules of carbon dioxide have been released, which accounts for the
six carbons in the starting glucose molecule. The high-energy NADH and
FADH) will be used in the last stage of aerobic respiration to produce
additional ATP molecules.
Electron Transport Chain/Oxidative Phosphorylation
You have just read about two pathways in glucose catabolism—glycolysis
and the citric acid cycle—that generate ATP. Most of the ATP generated
during the aerobic catabolism of glucose, however, is not generated directly
from these pathways. Rather, it derives from a process that begins with
passing electrons through a series of chemical reactions to a final electron
acceptor, oxygen. These reactions take place in specialized protein
complexes located in the inner membrane of the mitochondria. The energy
of the electrons is harvested and used to generate a electrochemical gradient
of hydrogen ions across the inner mitochondrial membrane. The potential
energy of this gradient is used to generate ATP by providing the energy to
add phosphate groups to ADP molecules. The entirety of this process is
called oxidative phosphorylation, as oxygen is required as the terminal
electron acceptor and phosphate groups are added to ADP molecules.
The electron transport chain ([link]a) is the last component of aerobic
respiration and is the only part of metabolism that uses atmospheric oxygen.
Oxygen continuously diffuses into plants for this purpose. In animals,
oxygen enters the body through the respiratory system. Electron transport is
a series of chemical reactions that resembles a bucket brigade in that
electrons are passed rapidly from one component to the next, to the
endpoint of the chain where oxygen is the final electron acceptor and water
is produced. There are four complexes composed of proteins, labeled I
through IV in [link]c, and the aggregation of these four complexes, together
with associated mobile, accessory electron carriers, is called the electron
transport chain. The electron transport chain is present in multiple copies
in the inner mitochondrial membrane of eukaryotes and in the plasma
membrane of prokaryotes. In each transfer of an electron through the
electron transport chain, the electron loses energy, but with some transfers,
the energy is stored as potential energy by using it to pump hydrogen ions
across the inner mitochondrial membrane into the intermembrane space,
creating an electrochemical gradient.
Note:
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(a) The electron transport chain is a set of molecules that supports a
series of oxidation-reduction reactions. (b) ATP synthase is a
complex, molecular machine that uses an H® gradient to regenerate
ATP from ADP. (c) An overview of the entire process.
MUN
Electrons from NADH and FADH) are passed to protein complexes in the
electron transport chain. As they are passed from one complex to another
(there are a total of four), the electrons lose energy, and some of that energy
is used to pump hydrogen ions from the mitochondrial matrix into the
intermembrane space. In the fourth protein complex, the electrons are
accepted by oxygen, the terminal acceptor. The oxygen with its extra
electrons then combines with two hydrogen ions, further enhancing the
electrochemical gradient, to form water. If there were no oxygen present in
the mitochondrion, the electrons could not be removed from the system, and
the entire electron transport chain would back up and stop. The
mitochondria would be unable to generate new ATP in this way, and the cell
would ultimately die from lack of energy. This is the reason we must
breathe to draw in new oxygen.
In the electron transport chain, the free energy from the series of reactions
just described is used to pump hydrogen ions (via active transport) across
the membrane. The uneven distribution of H* ions across the membrane
establishes an electrochemical gradient, owing to the H* ions’ positive
charge and their higher concentration on one side of the membrane.
Hydrogen ions diffuse through the inner membrane through a membrane
protein called ATP synthase ((link]b). This complex protein acts as a tiny
generator, turned by the force of the hydrogen ions diffusing through it,
down their electrochemical gradient from the intermembrane space, where
there are many mutually repelling hydrogen ions to the matrix, where there
are few. The turning of the parts of this molecular machine regenerate ATP
from ADP and phosphate.
Chemiosmosis (({link]c) is used to generate 90 percent of the ATP made
during aerobic glucose catabolism. The result of the reactions is the
production of ATP from the energy of the electrons removed from hydrogen
atoms. These atoms were originally part of a glucose molecule. At the end
of the electron transport system, the electrons are used to reduce an oxygen
molecule to oxygen ions. The extra electrons on the oxygen ions attract
hydrogen ions (protons) from the surrounding medium, and water is
formed.
ATP Yield
The number of ATP molecules generated from the catabolism of glucose
varies. In general, processing of each NADH yields approximately 3 ATP
and each FADH) yields approximately 2 ATP. Overall, a total of 10 NADH
and 2 FADH)> were produced in glycolysis, transition reaction, and the citric
acid cycle per glucose molecule. This results in the production of
approximately 34 ATP. Remember, that two additional ATP were produced
directly in both glycolysis and the citric acid cycle, resulting in a total yield
of 38 ATP per glucose. This represents an efficiency of approximately 35%,
with the remaining energy potential lost as heat or other products.
Note:
Careers in Action
Mitochondrial Disease Physician
What happens when the critical reactions of cellular respiration do not
proceed correctly? Mitochondrial diseases are genetic disorders of
metabolism. Mitochondrial disorders can arise from mutations in nuclear
or mitochondrial DNA, and they result in the production of less energy
than is normal in body cells. Symptoms of mitochondrial diseases can
include muscle weakness, lack of coordination, stroke-like episodes, and
loss of vision and hearing. Most affected people are diagnosed in
childhood, although there are some adult-onset diseases. Identifying and
treating mitochondrial disorders is a specialized medical field. The
educational preparation for this profession requires a college education,
followed by medical school with a specialization in medical genetics.
Medical geneticists can be board certified by the American Board of
Medical Genetics and go on to become associated with professional
organizations devoted to the study of mitochondrial disease, such as the
Mitochondrial Medicine Society and the Society for Inherited Metabolic
Disease.
Section Summary
The citric acid cycle is a series of chemical reactions that removes high-
energy electrons and uses them in the electron transport chain to generate
ATP. One molecule of ATP (or an equivalent) is produced per each turn of
the cycle.
The electron transport chain is the portion of aerobic respiration that uses
free oxygen as the final electron acceptor for electrons removed from the
intermediate compounds in glucose catabolism. The electrons are passed
through a series of chemical reactions, with a small amount of free energy
used at three points to transport hydrogen ions across the membrane. This
contributes to the gradient used in chemiosmosis. As the electrons are
passed from NADH or FADH> down the electron transport chain, they lose
energy. The products of the electron transport chain are water and ATP. A
number of intermediate compounds can be diverted into the anabolism of
other biochemical molecules, such as nucleic acids, non-essential amino
acids, sugars, and lipids. These same molecules, except nucleic acids, can
serve as energy sources for the glucose pathway.
Multiple Choice
Exercise:
Problem:What do the electrons added to NAD* do?
a. They become part of a fermentation pathway.
b. They go to another pathway for ATP production.
c. They energize the entry of the acetyl group into the citric acid
cycle.
d. They are converted into NADP.
Solution:
B
Exercise:
Problem: What provides the energy for ATP synthase?
a. the movement of electrons
b. the movement of hydrogen atoms
c. the movement of hydrogen ions
d. the movement of glucose
e. the movement of carbon dioxide
Solution:
C
Free Response
Exercise:
Problem:
We inhale oxygen when we breathe and exhale carbon dioxide. What
is the oxygen used for and where does the carbon dioxide come from?
Solution:
The oxygen we inhale is the final electron acceptor in the electron
transport chain and allows aerobic respiration to proceed, which is the
most efficient pathway for harvesting energy in the form of ATP from
food molecules. The carbon dioxide we breathe out is formed during
the transition reaction and the citric acid cycle when the bonds in
carbon compounds are broken.
Glossary
acetyl CoA
the combination of an acetyl group derived from pyruvic acid and
coenzyme A which is made from pantothenic acid (a B-group vitamin)
ATP synthase
a membrane-embedded protein complex that regenerates ATP from
ADP with energy from protons diffusing through it
chemiosmosis
the movement of hydrogen ions down their electrochemical gradient
across a membrane through ATP synthase to generate ATP
citric acid cycle
a series of enzyme-catalyzed chemical reactions of central importance
in all living cells that harvests the energy in carbon-carbon bonds of
sugar molecules to generate ATP; the citric acid cycle is an aerobic
metabolic pathway because it requires oxygen in later reactions to
proceed
electron transport chain
a series of four large, multi-protein complexes embedded in the inner
mitochondrial membrane that accepts electrons from donor compounds
and harvests energy from a series of chemical reactions to generate a
hydrogen ion gradient across the membrane
oxidative phosphorylation
the production of ATP by the transfer of electrons down the electron
transport chain to create a proton gradient that is used by ATP synthase
to add phosphate groups to ADP molecules
Fermentation
By the end of this section, you will be able to:
e Discuss the fundamental process of fermentation in human and and
yeast cells, including the final products of fermentation
In aerobic respiration, the final electron acceptor is an oxygen molecule, O>.
If aerobic respiration occurs, then ATP will be produced using the energy of
the high-energy electrons carried by NADH or FADH+s to the electron
transport chain. If aerobic respiration does not occur, NADH must be
reoxidized to NAD* for reuse as an electron carrier for glycolysis to
continue. How is this done? Humans use an organic molecule
(pyruvate/pyruvic acid)as the final electron acceptor. Processes that use an
organic molecule to regenerate NAD* from NADH are collectively referred
to as fermentation.
Lactic Acid Fermentation
The fermentation method used by animals and some bacteria like those in
yogurt is lactic acid fermentation ({link]). This occurs routinely in
mammalian red blood cells and in skeletal muscle that has insufficient
oxygen supply to allow aerobic respiration to continue (that is, in muscles
used to the point of fatigue). In muscles, lactic acid produced by
fermentation must be removed by the blood circulation and brought to the
liver for further metabolism. The chemical reaction of lactic acid
fermentation is the following:
Equation:
Pyruvic acid + NADH + lactic acid + NAD*
Note:
Art Connection
Lactic Acid Fermentation
2 Pyruvate
2 Lactate
:
E
i
Lactic acid fermentation is common in muscles
that have become exhausted by use.
Alcohol Fermentation
Another familiar fermentation process is alcohol fermentation ([Link]),
which produces ethanol, an alcohol. The alcohol fermentation reaction is
the following:
Pyruvic acid ———_—» CO, + Acetaldehyde
Acetaldehyde Ethanol
NADH NAD*
The reaction resulting in alcohol
fermentation is shown.
In the first reaction, a carboxyl group is removed from pyruvic acid,
releasing carbon dioxide as a gas. The loss of carbon dioxide reduces the
molecule by one carbon atom, making acetaldehyde. The second reaction
removes an electron from NADH, forming NAD* and producing ethanol
from the acetaldehyde, which accepts the electron. The fermentation of
pyruvic acid by yeast produces the ethanol found in alcoholic beverages
({link]). If the carbon dioxide produced by the reaction is not vented from
the fermentation chamber, for example in beer and sparkling wines, it
remains dissolved in the medium until the pressure is released. Ethanol
above 12 percent is toxic to yeast, so natural levels of alcohol in wine occur
at a maximum of 12 percent.
Fermentation of grape juice to
make wine produces CO> as a
byproduct. Fermentation tanks
have valves so that pressure
inside the tanks can be released.
Section Summary
If NADH cannot be metabolized through aerobic respiration, another
electron acceptor is used. Most organisms will use some form of
fermentation to accomplish the regeneration of NAD”, ensuring the
continuation of glycolysis. The regeneration of NAD" in fermentation is not
accompanied by ATP production; therefore, the potential for NADH to
produce ATP using an electron transport chain is not utilized.
Review Questions
Exercise:
Problem:
Which of the following fermentation methods can occur in animal
skeletal muscles deprived of oxygen?
a. lactic acid fermentation
b. alcohol fermentation
c. mixed acid fermentation
d. propionic fermentation
Solution:
A
Free Response
Exercise:
Problem:
When muscle cells run out of oxygen, what happens to the potential
for energy extraction from sugars and what pathways do the cell use?
Solution:
Without oxygen, the transition, the citric acid cycle, and the electron
transport chain stop, so ATP is no longer generated through this
mechanism, which extracts the greatest amount of energy from a sugar
molecule. In addition, NADH accumulates, preventing glycolysis from
going forward because of an absence of NAD”. Lactic acid
fermentation uses the electrons in NADH to generate lactic acid from
pyruvate, which allows glycolysis to continue and thus a smaller
amount of ATP can be generated by the cell (2 versus 38 ATP per
glucose).
Glossary
anaerobic cellular respiration
the use of an electron acceptor other than oxygen to complete
metabolism using electron transport-based chemiosmosis
fermentation
the steps that follow the partial oxidation of glucose via glycolysis to
regenerate NAD"; occurs in the absence of oxygen and uses an organic
compound as the final electron acceptor
Introduction to the Cardiovascular System - Blood
class="introduction"
Blood Cells
A single
drop of
blood
contains
millions of
red blood
cells, white
blood cells,
and
platelets.
One of each
type is
shown here
(from left
to right)
isolated
from a
scanning
electron
micrograph
Note:
Chapter Objectives
After studying this chapter, you will be able to:
Identify the primary functions of blood, its fluid and cellular
components, and its physical characteristics
Identify the most important proteins and other solutes present in blood
plasma
Describe the formation of the formed element components of blood
Discuss the structure and function of red blood cells and hemoglobin
Explain the significance of AB and Rh blood groups in blood
transfusions
Discuss a variety of blood disorders
The human body needs blood to deliver nutrients to and remove wastes
from our trillions of cells. The heart pumps blood throughout the body in a
network of blood vessels. Together, these three components—blood, heart,
and vessels—makes up the cardiovascular system. This chapter focuses on
the medium of transport: blood.
An Overview of Blood
By the end of this section, you will be able to:
e Identify the primary functions of blood in transportation, defense, and
maintenance of homeostasis
e Name the fluid component of blood and the three major types of
formed elements, and identify their relative proportions in a blood
sample
e Discuss the unique physical characteristics of blood
e Identify the composition of blood plasma, including its most important
solutes and plasma proteins
Blood is a connective tissue. Like all connective tissues, it is made up of
cellular elements and an extracellular matrix. The cellular elements—
referred to as the formed elements— include red blood cells (RBCs),
white blood cells (WBCs), and cell fragments called platelets. The
extracellular matrix, called plasma, makes blood unique among connective
tissues because it is fluid. This fluid, which is mostly water, perpetually
suspends the formed elements and enables them to circulate throughout the
body within the cardiovascular system.
Functions of Blood
The primary function of blood is to deliver oxygen and nutrients to and
remove wastes from body cells, but that is only the beginning of the story.
The specific functions of blood also include defense, distribution of heat,
and maintenance of homeostasis.
Transportation
Nutrients from the foods you eat are absorbed in the digestive tract. Most of
these travel in the bloodstream directly to the liver, where they are
processed and released back into the bloodstream for delivery to body cells.
Oxygen from the air you breathe diffuses into the blood, which moves from
the lungs to the heart, which then pumps it out to the rest of the body.
Moreover, endocrine glands scattered throughout the body release their
products, called hormones, into the bloodstream, which carries them to
distant target cells. Blood also picks up cellular wastes and byproducts, and
transports them to various organs for removal. For instance, blood moves
carbon dioxide to the lungs for exhalation from the body, and various waste
products are transported to the kidneys and liver for excretion from the
body in the form of urine or bile.
Defense
Many types of WBCs protect the body from external threats, such as
disease-causing bacteria that have entered the bloodstream in a wound.
Other WBCs seek out and destroy internal threats, such as cells with
mutated DNA that could multiply to become cancerous, or body cells
infected with viruses.
When damage to the vessels results in bleeding, blood platelets and certain
proteins dissolved in the plasma, the fluid portion of the blood, interact to
block the ruptured areas of the blood vessels involved. This protects the
body from further blood loss.
Maintenance of Homeostasis
Recall that body temperature is regulated via a classic negative-feedback
loop. If you were exercising on a warm day, your rising core body
temperature would trigger several homeostatic mechanisms, including
increased transport of blood from your core to your body periphery, which
is typically cooler. As blood passes through the vessels of the skin, heat
would be dissipated to the environment, and the blood returning to your
body core would be cooler. In contrast, on a cold day, blood is diverted
away from the skin to maintain a warmer body core. In extreme cases, this
may result in frostbite.
Blood also helps to maintain the chemical balance of the body. Proteins and
other compounds in blood act as buffers, which thereby help to regulate the
pH of body tissues. Blood also helps to regulate the water content of body
cells.
Composition of Blood
You have probably had blood drawn from a superficial vein in your arm,
which was then sent to a lab for analysis. Some of the most common blood
tests—for instance, those measuring lipid or glucose levels in plasma—
determine which substances are present within blood and in what quantities.
Other blood tests check for the composition of the blood itself, including
the quantities and types of formed elements.
One such test, called a hematocrit, measures the percentage of RBCs,
clinically known as erythrocytes, in a blood sample. It is performed by
spinning the blood sample in a specialized centrifuge, a process that causes
the heavier elements suspended within the blood sample to separate from
the lightweight, liquid plasma ({link]). Because the heaviest elements in
blood are the erythrocytes, these settle at the very bottom of the hematocrit
tube. Located above the erythrocytes is a pale, thin layer composed of the
remaining formed elements of blood. These are the WBCGs, clinically
known as leukocytes, and the platelets, cell fragments also called
thrombocytes. This layer is referred to as the buffy coat because of its
color; it normally constitutes less than 1 percent of a blood sample. Above
the buffy coat is the blood plasma, normally a pale, straw-colored fluid,
which constitutes the remainder of the sample.
In normal blood, about 45 percent of a sample is erythrocytes. The
hematocrit of any one sample can vary significantly, however, about 36—50
percent, according to gender and other factors. Normal hematocrit values
for females range from 37 to 47, with a mean value of 41; for males,
hematocrit ranges from 42 to 52, with a mean of 47. The percentage of
other formed elements, the WBCs and platelets, is extremely small so it is
not normally considered with the hematocrit. So the mean plasma
percentage is the percent of blood that is not erythrocytes: for females, it is
approximately 59 (or 100 minus 41), and for males, it is approximately 53
(or 100 minus 47).
Composition of Blood
Plasma:
- Water, proteins,
nutrients, hormones,
etc.
Buffy coat:
- White blood cells,
platelets
Hematocrit:
- Red blood cells
Normal Blood: Anemia: Polycythemia:
Q_ 37%-47% hematocrit Depressed Elevated
O" 42%-52% hematocrit hematocrit % hematocrit %
The cellular elements of blood include a
vast number of erythrocytes and
comparatively fewer leukocytes and
platelets. Plasma is the fluid in which the
formed elements are suspended. A sample
of blood spun in a centrifuge reveals that
plasma is the lightest component. It floats
at the top of the tube separated from the
heaviest elements, the erythrocytes, by a
buffy coat of leukocytes and platelets.
Hematocrit is the percentage of the total
sample that is comprised of erythrocytes.
Depressed and elevated hematocrit levels
are shown for comparison.
Characteristics of Blood
When you think about blood, the first characteristic that probably comes to
mind is its color. Blood that has just taken up oxygen in the lungs is bright
red, and blood that has released oxygen in the tissues is a more dusky (i.e
bluish) red. This is because hemoglobin is a pigment that changes color,
depending upon the degree of oxygen saturation.
The normal temperature of blood is slightly higher than normal body
temperature—about 38 °C (or 100.4 °F), compared to 37 °C (or 98.6 °F) for
an internal body temperature reading, although daily variations of 0.5 °C
are normal. Although the surface of blood vessels is relatively smooth, as
blood flows through them, it experiences some friction and resistance,
especially as vessels age and lose their elasticity, thereby producing heat.
This accounts for its slightly higher temperature.
The pH of blood averages about 7.4; however, it can range from 7.35 to
7.45 in a healthy person. Blood is therefore somewhat more basic (alkaline)
on a chemical scale than pure water, which has a pH of 7.0. Blood contains
numerous buffers that actually help to regulate pH.
Blood constitutes approximately 8 percent of adult body weight. Adult
males typically average about 5 to 6 liters of blood. Females average 4—5
liters.
Blood Plasma
Like other fluids in the body, plasma is composed primarily of water: In
fact, it is about 92 percent water. Dissolved or suspended within this water
is a mixture of substances, most of which are proteins. There are literally
hundreds of substances dissolved or suspended in the plasma, although
many of them are found only in very small quantities.
Plasma Proteins
About 7 percent of the volume of plasma—nearly all that is not water—is
made of proteins. These include several plasma proteins (proteins that are
unique to the plasma), plus a much smaller number of regulatory proteins,
including enzymes and some hormones. The major components of plasma
are summarized in [link].
The three major groups of plasma proteins are as follows:
e Albumin is the most abundant of the plasma proteins. Manufactured
by the liver, albumin molecules serve as binding proteins—transport
vehicles for fatty acids and steroid hormones. Recall that lipids are
hydrophobic; however, their binding to albumin enables their transport
in the watery plasma. Albumin is also the most significant contributor
to the osmotic pressure of blood; that is, its presence holds water inside
the blood vessels and draws water from the tissues, across blood vessel
walls, and into the bloodstream. This in turn helps to maintain both
blood volume and blood pressure. Albumin normally accounts for
approximately 54 percent of the total plasma protein content, in
clinical levels of 3.5—5.0 g/dL blood.
e The second most common plasma proteins are the globulins. A
heterogeneous group, there are three main subgroups known as alpha,
beta, and gamma globulins. The alpha and beta globulins transport
iron, lipids, and the fat-soluble vitamins A, D, E, and K to the cells;
like albumin, they also contribute to osmotic pressure. The gamma
globulins are proteins involved in immunity and are better known as an
antibodies or immunoglobulins. Although other plasma proteins are
produced by the liver, immunoglobulins are produced by specialized
leukocytes known as plasma cells. Globulins make up approximately
38 percent of the total plasma protein volume, in clinical levels of 1.0—
1.5 g/dL blood.
e The least abundant plasma protein is fibrinogen. Like albumin and the
alpha and beta globulins, fibrinogen is produced by the liver. It is
essential for blood clotting, a process described later in this chapter.
Fibrinogen accounts for about 7 percent of the total plasma protein
volume, in clinical levels of 0.2—0.45 g/dL blood.
Other Plasma Solutes
In addition to proteins, plasma contains a wide variety of other substances.
These include various electrolytes, such as sodium, potassium, and calcium
ions; dissolved gases, such as oxygen, carbon dioxide, and nitrogen; various
organic nutrients, such as vitamins, lipids, glucose, and amino acids; and
metabolic wastes. All of these nonprotein solutes combined contribute
approximately 1 percent to the total volume of plasma.
Note:
Career Connection
Phlebotomy and Medical Lab Technology
Phlebotomists are professionals trained to draw blood (phleb- = “a blood
vessel”; -tomy = “to cut”). When more than a few drops of blood are
required, phlebotomists perform a venipuncture, typically of a surface vein
in the arm. They perform a capillary stick on a finger, an earlobe, or the
heel of an infant when only a small quantity of blood is required. An
arterial stick is collected from an artery and used to analyze blood gases.
After collection, the blood may be analyzed by medical laboratories or
perhaps used for transfusions, donations, or research. While many allied
health professionals practice phlebotomy, the American Society of
Phlebotomy Technicians issues certificates to individuals passing a
national examination, and some large labs and hospitals hire individuals
expressly for their skill in phlebotomy.
Medical or clinical laboratories employ a variety of individuals in technical
positions:
e Medical technologists (MT), also known as clinical laboratory
technologists (CLT), typically hold a bachelor’s degree and
certification from an accredited training program. They perform a
wide variety of tests on various body fluids, including blood. The
information they provide is essential to the primary care providers in
determining a diagnosis and in monitoring the course of a disease and
response to treatment.
¢ Medical laboratory technicians (MLT) typically have an associate’s
degree but may perform duties similar to those of an MT.
e Medical laboratory assistants (MLA) spend the majority of their time
processing samples and carrying out routine assignments within the
lab. Clinical training is required, but a degree may not be essential to
obtaining a position.
Chapter Review
Blood is a fluid connective tissue critical to the transportation of nutrients,
gases, and wastes throughout the body; to defend the body against infection
and other threats; and to the homeostatic regulation of pH, temperature, and
other internal conditions. Blood is composed of formed elements—
erythrocytes, leukocytes, and cell fragments called platelets—and a fluid
extracellular matrix called plasma. More than 90 percent of plasma is water.
The remainder is mostly plasma proteins—mainly albumin, globulins, and
fibrinogen—and other dissolved solutes such as glucose, lipids, electrolytes,
and dissolved gases. Blood is also slightly alkaline, and its temperature is
slightly higher than normal body temperature.
Critical Thinking Questions
Exercise:
Problem:
Why would it be incorrect to refer to the formed elements as cells?
Solution:
The formed elements include erythrocytes and leukocytes, which are
cells (although mature erythrocytes do not have a nucleus); however,
the formed elements also include platelets, which are not true cells but
cell fragments.
Glossary
albumin
most abundant plasma protein, accounting for most of the osmotic
pressure of plasma
antibodies
(also, immunoglobulins or gamma globulins) antigen-specific proteins
produced by specialized B lymphocytes that protect the body by
binding to foreign objects such as bacteria and viruses
blood
liquid connective tissue composed of formed elements—erythrocytes,
leukocytes, and platelets—and a fluid extracellular matrix called
plasma; component of the cardiovascular system
buffy coat
thin, pale layer of leukocytes and platelets that separates the
erythrocytes from the plasma in a sample of centrifuged blood
fibrinogen
plasma protein produced in the liver and involved in blood clotting
formed elements
cellular components of blood; that is, erythrocytes, leukocytes, and
platelets
globulins
heterogeneous group of plasma proteins that includes transport
proteins, clotting factors, immune proteins, and others
hematocrit
(also, packed cell volume) volume percentage of erythrocytes in a
sample of centrifuged blood
immunoglobulins
(also, antibodies or gamma globulins) antigen-specific proteins
produced by specialized B lymphocytes that protect the body by
binding to foreign objects such as bacteria and viruses
packed cell volume (PCV)
(also, hematocrit) volume percentage of erythrocytes present in a
sample of centrifuged blood
plasma
in blood, the liquid extracellular matrix composed mostly of water that
circulates the formed elements and dissolved materials throughout the
cardiovascular system
platelets
(also, thrombocytes) one of the formed elements of blood that consists
of cell fragments broken off from megakaryocytes
red blood cells (RBCs)
(also, erythrocytes) one of the formed elements of blood that transports
oxygen
white blood cells (WBCs)
(also, leukocytes) one of the formed elements of blood that provides
defense against disease agents and foreign materials
Erythrocytes
By the end of this section, you will be able to:
¢ Describe the anatomy of erythrocytes
e Explain the composition and function of hemoglobin
The erythrocyte, commonly known as a red blood cell (or RBC), is by far
the most common formed element: A single drop of blood contains millions
of erythrocytes and just thousands of leukocytes. Specifically, males have
about 5.4 million erythrocytes per microliter (uL) of blood, and females
have approximately 4.8 million per pL. In fact, erythrocytes are estimated
to make up about 25 percent of the total cells in the body. As you can
imagine, they are quite small cells, with a mean diameter of only about 7-8
micrometers (um). The primary functions of erythrocytes are to pick up
inhaled oxygen from the lungs and transport it to the body’s tissues, and to
pick up some (about 24 percent) carbon dioxide waste at the tissues and
transport it to the lungs for exhalation. Erythrocytes remain within the
vascular network. Although leukocytes typically leave the blood vessels to
perform their defensive functions, movement of erythrocytes from the
blood vessels is abnormal.
Shape and Structure of Erythrocytes
As an erythrocyte matures in the red bone marrow, it extrudes its nucleus
and most of its other organelles. Lacking mitochondria, for example, they
rely on fermentation. This means that they do not utilize any of the oxygen
they are transporting, so they can deliver it all to the tissues. They also lack
endoplasmic reticula and do not synthesize proteins, so they are unable to
repair themselves. This is why the lifespan of a red blood cell is
approximately 115 days. However, during this time, the red blood cell has
traveled approximately 300 miles and made approximately 170,000 circuits
through the heart (http://www.uptodate.com/contents/red-blood-cell-
survival-normal-values-and-measurement).
Erythrocytes are biconcave disks; that is, they are plump at their periphery
and very thin in the center ({link]). Since they lack most organelles, there is
more interior space for the presence of the hemoglobin molecules that, as
you will see shortly, transport gases. The biconcave shape also provides a
greater surface area across which gas exchange can occur, relative to its
volume; a sphere of a similar diameter would have a lower surface area-to-
volume ratio. In the capillaries, the oxygen carried by the erythrocytes can
diffuse into the plasma and then through the capillary walls to reach the
cells, whereas some of the carbon dioxide produced by the cells as a waste
product diffuses into the capillaries to be picked up by the erythrocytes.
Capillary beds are extremely narrow, slowing the passage of the
erythrocytes and providing an extended opportunity for gas exchange to
occur. However, the space within capillaries can be so minute that, despite
their own small size, erythrocytes may have to fold in on themselves if they
are to make their way through. Fortunately, their structural proteins are
flexible, allowing them to bend over themselves to a surprising degree, then
spring back again when they enter a wider vessel.
Shape of Red Blood Cells
4
Erythrocytes are biconcave
discs with very shallow centers.
This shape optimizes the ratio
of surface area to volume,
facilitating gas exchange. It also
enables them to fold up as they
move through narrow blood
vessels.
Hemoglobin
Hemoglobin is a large molecule made up of proteins and iron. It consists of
four folded chains of a protein called globin, designated alpha 1 and 2, and
beta 1 and 2 ({link]a). Each of these globin molecules is bound to a red
pigment molecule called heme, which contains an ion of iron (Fe**)
({link]b).
Hemoglobin
8 chain 2
a chain 1
(a) A molecule of hemoglobin contains four globin
proteins, each of which is bound to one molecule of the
iron-containing pigment heme. (b) A single erythrocyte
can contain 300 million hemoglobin molecules, and thus
more than 1 billion oxygen molecules.
Each iron ion in the heme can bind to one oxygen molecule; therefore, each
hemoglobin molecule can transport four oxygen molecules. An individual
erythrocyte may contain about 300 million hemoglobin molecules, and
therefore can bind to and transport up to 1.2 billion oxygen molecules (see
[link]b).
In the lungs, hemoglobin picks up oxygen, which binds to the iron ions,
forming oxyhemoglobin. The bright red, oxygenated hemoglobin travels to
the body tissues, where it releases some of the oxygen molecules, becoming
darker red deoxyhemoglobin. Oxygen release depends on the need for
oxygen in the surrounding tissues, so hemoglobin rarely if ever leaves all of
its oxygen behind. In the capillaries, carbon dioxide enters the bloodstream.
About 76 percent dissolves in the plasma, some of it remaining as dissolved
COs, and the remainder forming bicarbonate ion. About 23—24 percent of it
binds to the amino acids in hemoglobin, forming a molecule known as
carbaminohemoglobin. From the capillaries, the hemoglobin carries
carbon dioxide back to the lungs, where it releases it for exchange of
oxygen.
In patients with insufficient hemoglobin, the tissues may not receive
sufficient oxygen, resulting in another form of anemia. In determining
oxygenation of tissues, the value of greatest interest in healthcare is the
percent saturation; that is, the percentage of hemoglobin sites occupied by
oxygen in a patient’s blood. Clinically this value is commonly referred to
simply as “percent sat.”
Percent saturation is normally monitored using a device known as a pulse
oximeter, which is applied to a thin part of the body, typically the tip of the
patient’s finger. The device works by sending two different wavelengths of
light (one red, the other infrared) through the finger and measuring the light
with a photodetector as it exits. Hemoglobin absorbs light differentially
depending upon its saturation with oxygen. The machine calibrates the
amount of light received by the photodetector against the amount absorbed
by the partially oxygenated hemoglobin and presents the data as percent
saturation. Normal pulse oximeter readings range from 95-100 percent.
Lower percentages reflect hypoxemia, or low blood oxygen. The term
hypoxia is more generic and simply refers to low oxygen levels. Oxygen
levels are also directly monitored from free oxygen in the plasma typically
following an arterial stick. When this method is applied, the amount of
oxygen present is expressed in terms of partial pressure of oxygen or simply
pO, and is typically recorded in units of millimeters of mercury, mm Hg.
Lifecycle of Erythrocytes
Production of erythrocytes in the marrow occurs at the staggering rate of
more than 2 million cells per second. For this production to occur, a number
of raw materials must be present in adequate amounts. These include the
same nutrients that are essential to the production and maintenance of any
cell, such as glucose, lipids, and amino acids. However, erythrocyte
production also requires several trace elements: iron, copper, zinc, and
several types of B vitamins.
Erythrocytes live up to 120 days in the circulation, after which the worn-out
cells are removed by a type of phagocytic cell called a macrophage,
located primarily within the bone marrow, liver, and spleen. The
components of the degraded erythrocytes’ hemoglobin are further
processed, with some being retained by the body and others being released
in the urine and feces.
The breakdown pigments formed from the destruction of hemoglobin can
be seen in a variety of situations. At the site of an injury, biliverdin from
damaged RBCs produces some of the dramatic colors associated with
bruising. With a failing liver, bilirubin cannot be removed effectively from
circulation and causes the body to assume a yellowish tinge associated with
jaundice. Stercobilins within the feces produce the typical brown color
associated with this waste. And the yellow of urine is associated with the
urobilins.
Disorders of Erythrocytes
The size, shape, and number of erythrocytes, and the number of hemoglobin
molecules can have a major impact on a person’s health. When the number
of RBCs or hemoglobin is deficient, the general condition is called anemia.
There are more than 400 types of anemia and more than 3.5 million
Americans suffer from this condition. Anemia can be broken down into
three major groups: those caused by blood loss, those caused by faulty or
decreased RBC production, and those caused by excessive destruction of
RBCs. The effects of the various anemias are widespread, because reduced
numbers of RBCs or hemoglobin will result in lower levels of oxygen being
delivered to body tissues. Since oxygen is required for tissue functioning,
anemia produces fatigue, lethargy, and an increased risk for infection. An
oxygen deficit in the brain impairs the ability to think clearly, and may
prompt headaches and irritability. Lack of oxygen leaves the patient short of
breath, even as the heart and lungs work harder in response to the deficit.
Anemias caused by faulty or decreased RBC production include sickle cell
anemia, iron deficiency anemia, vitamin deficiency anemia, and diseases of
the bone marrow and stem cells.
e A characteristic change in the shape of erythrocytes is seen in sickle
cell disease (also referred to as sickle cell anemia). A genetic disorder,
it is caused by production of an abnormal type of hemoglobin, called
hemoglobin S, which delivers less oxygen to tissues and causes
erythrocytes to assume a sickle (or crescent) shape, especially at low
oxygen concentrations ({link]). These abnormally shaped cells can
then become lodged in narrow capillaries because they are unable to
fold in on themselves to squeeze through, blocking blood flow to
tissues and causing a variety of serious problems from painful joints to
delayed growth and even blindness and cerebrovascular accidents
(strokes). Sickle cell anemia is a genetic condition particularly found
in individuals of African descent.
Sickle Cells
Sickle cell anemia is caused by
a mutation in one of the
hemoglobin genes. Erythrocytes
produce an abnormal type of
hemoglobin, which causes the
cell to take on a sickle or
crescent shape. (credit: Janice
Haney Carr)
e Iron deficiency anemia is the most common type and results when the
amount of available iron is insufficient to allow production of
sufficient heme. This condition can occur in individuals with a
deficiency of iron in the diet and is especially common in teens and
children as well as in vegans and vegetarians. Additionally, iron
deficiency anemia may be caused by either an inability to absorb and
transport iron or slow, chronic bleeding.
e Vitamin-deficient anemias generally involve insufficient vitamin B12
and folate.
o Pernicious anemia is caused by poor absorption of vitamin B12
and is often seen in patients with Crohn’s disease (a severe
intestinal disorder often treated by surgery), surgical removal of
the intestines or stomach (common in some weight loss
surgeries), intestinal parasites, and AIDS.
o Pregnancies, some medications, excessive alcohol consumption,
and some diseases such as celiac disease are also associated with
vitamin deficiencies. It is essential to provide sufficient folic acid
during the early stages of pregnancy to reduce the risk of
neurological defects, including spina bifida, a failure of the neural
tube to close.
e Assorted disease processes can also interfere with the production and
formation of RBCs and hemoglobin. If blood stem cells are defective
or replaced by cancer cells, there will be insufficient quantities of
RBCs produced.
o Aplastic anemia is the condition in which there are deficient
numbers of RBC stem cells. Aplastic anemia is often inherited, or
it may be triggered by radiation, medication, chemotherapy, or
infection.
o Thalassemia is an inherited condition typically occurring in
individuals from the Middle East, the Mediterranean, African, and
Southeast Asia, in which maturation of the RBCs does not
proceed normally. The most severe form is called Cooley’s
anemia.
o Lead exposure from industrial sources or even dust from paint
chips of iron-containing paints or pottery that has not been
properly glazed may also lead to destruction of the red marrow.
In contrast to anemia, an elevated RBC count is called polycythemia and is
detected in a patient’s elevated hematocrit. It can occur transiently in a
person who is dehydrated; when water intake is inadequate or water losses
are excessive, the plasma volume falls. As a result, the hematocrit rises. For
reasons mentioned earlier, a mild form of polycythemia is chronic but
normal in people living at high altitudes. Some elite athletes train at high
elevations specifically to induce this phenomenon. Finally, a type of bone
marrow disease called polycythemia vera (from the Greek vera = “true”)
causes an excessive production of immature erythrocytes. Polycythemia
vera can dangerously elevate the viscosity of blood, raising blood pressure
and making it more difficult for the heart to pump blood throughout the
body. It is a relatively rare disease that occurs more often in men than
women, and is more likely to be present in elderly patients those over 60
years of age.
Chapter Review
The most abundant formed elements in blood, erythrocytes are red,
biconcave disks packed with an oxygen-carrying compound called
hemoglobin. The hemoglobin molecule contains four globin proteins bound
to a pigment molecule called heme, which contains an ion of iron. In the
bloodstream, iron picks up oxygen in the lungs and drops it off in the
tissues; the amino acids in hemoglobin then transport carbon dioxide from
the tissues back to the lungs. Erythrocytes live approximately 115 days on
average, and thus must be continually replaced. Worn-out erythrocytes are
phagocytized by macrophages and their hemoglobin is broken down. The
breakdown products are recycled or removed as wastes. Anemia is a
deficiency of RBCs or hemoglobin, whereas polycythemia is an excess of
RBCs.
Review Questions
Exercise:
Problem:
Which of the following statements about mature, circulating
erythrocytes is true?
a. They have no nucleus.
b. They are packed with mitochondria.
c. They survive for an average of 4 days.
d. All of the above
Solution:
A
Exercise:
Problem:A molecule of hemoglobin
a. is shaped like a biconcave disk packed almost entirely with iron
b. contains four glycoprotein units studded with oxygen
c. consists of four globin proteins, each bound to a molecule of
heme
d. can carry up to 120 molecules of oxygen
Solution:
C
Exercise:
Problem:
The production of healthy erythrocytes depends upon the availability
of
a. Copper
b. zinc
c. vitamin By
d. copper, zinc, and vitamin By
Solution:
D
Exercise:
Problem:
Aging and damaged erythrocytes are removed from the circulation by
a. myeoblasts
b. monocytes
c. macrophages
d. mast cells
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
A young woman has been experiencing unusually heavy menstrual
bleeding for several years. She follows a strict vegan diet (no animal
foods). She is at risk for what disorder, and why?
Solution:
She is at risk for anemia, because her unusually heavy menstrual
bleeding results in excessive loss of erythrocytes each month. At the
same time, her vegan diet means that she does not have dietary sources
of heme iron. The non-heme iron she consumes in plant foods is not as
well absorbed as heme iron.
Glossary
anemia
deficiency of red blood cells or hemoglobin
bilirubin
yellowish bile pigment produced when iron is removed from heme and
is further broken down into waste products
biliverdin
green bile pigment produced when the non-iron portion of heme is
degraded into a waste product; converted to bilirubin in the liver
carbaminohemoglobin
compound of carbon dioxide and hemoglobin, and one of the ways in
which carbon dioxide is carried in the blood
deoxyhemoglobin
molecule of hemoglobin without an oxygen molecule bound to it
erythrocyte
(also, red blood cell) mature myeloid blood cell that is composed
mostly of hemoglobin and functions primarily in the transportation of
oxygen and carbon dioxide
ferritin
protein-containing storage form of iron found in the bone marrow,
liver, and spleen
globin
heme-containing globular protein that is a constituent of hemoglobin
heme
red, iron-containing pigment to which oxygen binds in hemoglobin
hemoglobin
oxygen-carrying compound in erythrocytes
hemosiderin
protein-containing storage form of iron found in the bone marrow,
liver, and spleen
hypoxemia
below-normal level of oxygen saturation of blood (typically <95
percent)
macrophage
phagocytic cell of the myeloid lineage; a matured monocyte
oxyhemoglobin
molecule of hemoglobin to which oxygen is bound
polycythemia
elevated level of hemoglobin, whether adaptive or pathological
reticulocyte
immature erythrocyte that may still contain fragments of organelles
sickle cell disease
(also, sickle cell anemia) inherited blood disorder in which
hemoglobin molecules are malformed, leading to the breakdown of
RBCs that take on a characteristic sickle shape
thalassemia
inherited blood disorder in which maturation of RBCs does not
proceed normally, leading to abnormal formation of hemoglobin and
the destruction of RBCs
transferrin
plasma protein that binds reversibly to iron and distributes it
throughout the body
Blood Typing and Transfusions
By the end of this section, you will be able to:
e Describe the two basic physiological consequences of transfusion of
incompatible blood
e Compare and contrast ABO and Rh blood groups
Identify which blood groups may be safely transfused into patients with
different ABO types
e Discuss the pathophysiology of hemolytic disease of the newborn
Blood transfusions in humans were risky procedures until the discovery of the
major human blood groups by Karl Landsteiner, an Austrian biologist and
physician, in 1900. Until that point, physicians did not understand that death
sometimes followed blood transfusions, when the type of donor blood infused into
the patient was incompatible with the patient’s own blood. Blood groups are
determined by the presence or absence of specific marker molecules, called
antigens, on the plasma membranes of erythrocytes. With their discovery, it became
possible for the first time to match patient-donor blood types and prevent
transfusion reactions and deaths.
Antigens, Antibodies, and Transfusion Reactions
Antigens are substances that the body does not recognize as belonging to the “self”
and that therefore trigger a defensive response from the leukocytes (WBC) of the
immune system. Here, we will focus on the role of immunity, antigens, and
antibodies in blood transfusion reactions.
Antigens are generally large proteins, but may include other classes of organic
molecules, including carbohydrates, lipids, and nucleic acids. Following an
infusion of incompatible blood, erythrocytes with foreign antigens appear in the
bloodstream and trigger an immune response. Proteins called antibodies
(immunoglobulins), which are produced by certain B lymphocytes called plasma
cells, attach to the antigens on the plasma membranes of the infused erythrocytes
and cause them to stick to one another (i.e. agglutinate). The clumps of
erythrocytes block small blood vessels throughout the body, depriving tissues of
oxygen and nutrients. As the erythrocyte clumps are degraded, in a process called
hemolysis, their hemoglobin is released into the bloodstream. This hemoglobin
travels to the kidneys, which are responsible for filtration of the blood. However,
the load of hemoglobin released can easily overwhelm the kidney’s capacity to
clear it, and the patient can quickly develop kidney failure.
More than 50 antigens have been identified on erythrocyte membranes, but the
most significant in terms of their potential harm to patients are classified in two
groups: the ABO blood group and the Rh blood group.
The ABO Blood Group
Although the ABO blood group name consists of three letters, ABO blood typing
designates the presence or absence of just two antigens, A and B. Both are
glycoproteins. People whose erythrocytes have A antigens on their erythrocyte
membrane surfaces are designated blood type A, and those whose erythrocytes
have B antigens are blood type B. People can also have both A and B antigens on
their erythrocytes, in which case they are blood type AB. People with neither A nor
B antigens are designated blood type O. ABO blood types are genetically
determined.
Normally the body must be exposed to a foreign antigen before an antibody can be
produced. This is not the case for the ABO blood group. Individuals with type A
blood—without any prior exposure to incompatible blood—have preformed
antibodies to the B antigen circulating in their blood plasma. These antibodies,
referred to as anti-B antibodies, will cause agglutination and hemolysis if they ever
encounter erythrocytes with B antigens. Similarly, an individual with type B blood
has pre-formed anti-A antibodies. Individuals with type AB blood, which has both
antigens, do not have preformed antibodies to either of these. People with type O
blood lack antigens A and B on their erythrocytes, but both anti-A and anti-B
antibodies circulate in their blood plasma.
Rh Blood Groups
The Rh blood group is classified according to the presence or absence of a second
erythrocyte antigen identified as Rh. (It was first discovered in a type of primate
known as a rhesus macaque, which is often used in research, because its blood is
similar to that of humans.) Although dozens of Rh antigens have been identified,
only one, designated D, is clinically important. Those who have the Rh D antigen
present on their erythrocytes—about 85 percent of Americans—are described as
Rh positive (Rh*) and those who lack it are Rh negative (Rh ). Note that the Rh
group is distinct from the ABO group, so any individual, no matter their ABO
blood type, may have or lack this Rh antigen. When identifying a patient’s blood
type, the Rh group is designated by adding the word positive or negative to the
ABO type. For example, A positive (A*) means ABO group A blood with the Rh
antigen present, and AB negative (AB ) means ABO group AB blood without the
Rh antigen.
[link] summarizes the distribution of the ABO and Rh blood types within the
United States.
Summary of ABO and Rh Blood Types within the United States
Blood African- Asian- Caucasian- Latino/Latina-
Type Americans Americans Americans Americans
At 24 27 33 29
AS 2 0.5 7 2
Bt 18 25 9 9
B 1 0.4 2 1
AB* 4 7 3 2
AB™ 0.3 0.1 1 0.2
O* 47 39 a7 53
O- 4 1 8 4
In contrast to the ABO group antibodies, which are preformed, antibodies to the Rh
antigen are produced only in Rh’ individuals after exposure to the antigen. This
process, called sensitization, occurs following a transfusion with Rh-incompatible
blood or, more commonly, with the birth of an Rh* baby to an Rh” mother.
Problems are rare in a first pregnancy, since the baby’s Rh’ cells rarely cross the
placenta (the organ of gas and nutrient exchange between the baby and the mother).
However, during or immediately after birth, the Rh’ mother can be exposed to the
baby’s Rh’ cells ({link]). Research has shown that this occurs in about 13-14
percent of such pregnancies. After exposure, the mother’s immune system begins
to generate anti-Rh antibodies. If the mother should then conceive another Rh*
baby, the Rh antibodies she has produced can cross the placenta into the fetal
bloodstream and destroy the fetal RBCs. This condition, known as hemolytic
disease of the newborn (HDN) or erythroblastosis fetalis, may cause anemia in
mild cases, but the agglutination and hemolysis can be so severe that without
treatment the fetus may die in the womb or shortly after birth.
Erythroblastosis Fetalis
Placenta
Umbilical
artery/vein
(infant)
Placental arteriole (maternal)
Placental venule (maternal)
II
Maternal blood pool
First exposure:
Birth of first Rh* infant
Embryonic chorion
(isolates fetal blood
from maternal
blood pool)
(1) During birth, Rh*
fetal erythrocytes
leak into maternal
Infant blood
blood after breakage
of the embryonic
chorion, which
normally isolates
the fetal and
maternal blood.
Rh* infant
erythrocyte
a Embryonic
chorion
| Rh” maternal
|
blood Maternal B cells
are activated by
1 \ Maternal Rh the Rh antigen
\ \ antibody and produce large
\ amounts of
anti-Rh antibodies.
. NN Second exposure:
oN Rh* fetus
f Infant blood
Rh antibody titer in
mother’s blood is
elevated after first
exposure.
Rh* infant
erythrocyte
Embryonic
chorion Rh antibodies are
small enough
to cross the
Rh" maternal : ‘
blood embryonic chorion
and attack the fetal
Maternal Rh erythrocytes.
antibody
The first exposure of an Rh” mother to Rh* erythrocytes during
pregnancy induces sensitization. Anti-Rh antibodies begin to
circulate in the mother’s bloodstream. A second exposure occurs with
a subsequent pregnancy with an Rh” fetus in the uterus. Maternal
anti-Rh antibodies may cross the placenta and enter the fetal
bloodstream, causing agglutination and hemolysis of fetal
erythrocytes.
A drug known as RhoGAM, short for Rh immune globulin, can temporarily
prevent the development of Rh antibodies in the Rh” mother, thereby averting this
potentially serious disease for the fetus. RhaoGAM antibodies destroy any fetal Rh*
erythrocytes that may cross the placental barrier. RhoGAM is normally
administered to Rh mothers during weeks 26-28 of pregnancy and within 72
hours following birth. It has proven remarkably effective in decreasing the
incidence of HDN. Earlier we noted that the incidence of HDN in an Rh*
subsequent pregnancy to an Rh mother is about 13-14 percent without preventive
treatment. Since the introduction of RhoGAM in 1968, the incidence has dropped
to about 0.1 percent in the United States.
Determining ABO Blood Types
Clinicians are able to determine a patient’s blood type quickly and easily using
commercially prepared antibodies. An unknown blood sample is allocated into
separate wells. Into one well a small amount of anti-A antibody is added, and to
another a small amount of anti-B antibody. If the antigen is present, the antibodies
will cause visible agglutination of the cells ({link]). The blood should also be tested
for Rh antibodies.
Cross Matching Blood Types
SAMPLE AB0+D
Bathe oy - 4 ¥
Le i
a Be att ot x 3
a ie ,
Agglutinated ms aa 3s
RBCs = Ape Se i
Anti-A Anti-B Anti-D
This sample of a commercially produced
“bedside” card enables quick typing of both a
recipient’s and donor’s blood before transfusion.
The card contains three reaction sites or wells.
One is coated with an anti-A antibody, one with
an anti-B antibody, and one with an anti-D
antibody (tests for the presence of Rh factor D).
Mixing a drop of blood and saline into each well
enables the blood to interact with a preparation
of type-specific antibodies, also called anti-
seras. Agglutination of RBCs in a given site
indicates a positive identification of the blood
antigens, in this case A and Rh antigens for
blood type A*. For the purpose of transfusion,
the donor’s and recipient’s blood types must
match.
ABO Transfusion Protocols
To avoid transfusion reactions, it is best to transfuse only matching blood types;
that is, a type B* recipient should ideally receive blood only from a type B* donor
and so on. That said, in emergency situations, when acute hemorrhage threatens the
patient’s life, there may not be time for cross matching to identify blood type. In
these cases, blood from a universal donor—an individual with type O° blood—
may be transfused. Recall that type O erythrocytes do not display A or B antigens.
Thus, anti-A or anti-B antibodies that might be circulating in the patient’s blood
plasma will not encounter any erythrocyte surface antigens on the donated blood
and therefore will not be provoked into a response. One problem with this
designation of universal donor is if the O" individual had prior exposure to Rh
antigen, Rh antibodies may be present in the donated blood. Also, introducing type
O blood into an individual with type A, B, or AB blood will nevertheless introduce
antibodies against both A and B antigens, as these are always circulating in the type
O blood plasma. This may cause problems for the recipient, but because the
volume of blood transfused is much lower than the volume of the patient’s own
blood, the adverse effects of the relatively few infused plasma antibodies are
typically limited. Rh factor also plays a role. If Rh individuals receiving blood
have had prior exposure to Rh antigen, antibodies for this antigen may be present in
the blood and trigger agglutination to some degree. Although it is always preferable
to cross match a patient’s blood before transfusing, in a true life-threatening
emergency situation, this is not always possible, and these procedures may be
implemented.
A patient with blood type AB" is known as the universal recipient. This patient
can theoretically receive any type of blood, because the patient’s own blood—
having both A and B antigens on the erythrocyte surface—does not produce anti-A
or anti-B antibodies. In addition, an Rh” patient can receive both Rh* and Rh™
blood. However, keep in mind that the donor’s blood will contain circulating
antibodies, again with possible negative implications. [link] summarizes the blood
types and compatibilities.
At the scene of multiple-vehicle accidents, military engagements, and natural or
human-caused disasters, many victims may suffer simultaneously from acute
hemorrhage, yet type O blood may not be immediately available. In these
circumstances, medics may at least try to replace some of the volume of blood that
has been lost. This is done by intravenous administration of a saline solution that
provides fluids and electrolytes in proportions equivalent to those of normal blood
plasma. Research is ongoing to develop a safe and effective artificial blood that
would carry out the oxygen-carrying function of blood without the RBCs, enabling
transfusions in the field without concern for incompatibility. These blood
substitutes normally contain hemoglobin- as well as perfluorocarbon-based oxygen
Carriers.
ABO Blood Group
Blood Type
Red Blood
Cell Type
sll? ye
ea ou oe et
Antibodies
in Plasma = Ye “™ 4 \. 4N.
Anti-A and Anti-B
Antigens in
Red blood t f
Cell Aantigen B antigen A and B antigens None
Blood Types
, A, B, AB, O
ee (AB* is the Ois ie
E universal recipient) universal donor)
mergency
This chart summarizes the characteristics of
the blood types in the ABO blood group. See
the text for more on the concept of a universal
donor or recipient.
Chapter Review
Antigens are nonself molecules, usually large proteins, which provoke an immune
response. In transfusion reactions, antibodies attach to antigens on the surfaces of
erythrocytes and cause agglutination and hemolysis. ABO blood group antigens are
designated A and B. People with type A blood have A antigens on their
erythrocytes, whereas those with type B blood have B antigens. Those with AB
blood have both A and B antigens, and those with type O blood have neither A nor
B antigens. The blood plasma contains preformed antibodies against the antigens
not present on a person’s erythrocytes.
A second group of blood antigens is the Rh group, the most important of which is
Rh D. People with Rh’ blood do not have this antigen on their erythrocytes,
whereas those who are Rh* do. About 85 percent of Americans are Rh*. When a
woman who is Rh” becomes pregnant with an Rh* fetus, her body may begin to
produce anti-Rh antibodies. If she subsequently becomes pregnant with a second
Rh’ fetus and is not treated preventively with RhoGAM, the fetus will be at risk for
an antigen-antibody reaction, including agglutination and hemolysis. This is known
as hemolytic disease of the newborn.
Cross matching to determine blood type is necessary before transfusing blood,
unless the patient is experiencing hemorrhage that is an immediate threat to life, in
which case type O' blood may be transfused.
Review Questions
Exercise:
Problem:
The process in which antibodies attach to antigens, causing the formation of
masses of linked cells, is called
a. sensitization
b. coagulation
c. agglutination
d. hemolysis
Solution:
Cc
Exercise:
Problem:People with ABO blood type O
a. have both antigens A and B on their erythrocytes
b. lack both antigens A and B on their erythrocytes
c. have neither anti-A nor anti-B antibodies circulating in their blood
plasma
d. are considered universal recipients
Solution:
B
Exercise:
Problem:
Hemolytic disease of the newborn is a risk during a subsequent pregnancy in
which
a. a type AB mother is carrying a type O fetus
b. a type O mother is carrying a type AB fetus
c. an Rh* mother is carrying an Rh” fetus
d. an Rh” mother is carrying a second Rh’ fetus
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Following a motor vehicle accident, a patient is rushed to the emergency
department with multiple traumatic injuries, causing severe bleeding. The
patient’s condition is critical, and there is no time for determining his blood
type. What type of blood is transfused, and why?
Solution:
In emergency situations, blood type O' will be infused until cross matching
can be done. Blood type O is called the universal donor blood because the
erythrocytes have neither A nor B antigens on their surface, and the Rh factor
is negative.
Exercise:
Problem:
In preparation for a scheduled surgery, a patient visits the hospital lab for a
blood draw. The technician collects a blood sample and performs a test to
determine its type. She places a sample of the patient’s blood in two wells. To
the first well she adds anti-A antibody. To the second she adds anti-B
antibody. Both samples visibly agglutinate. Has the technician made an error,
or is this a normal response? If normal, what blood type does this indicate?
Solution:
The lab technician has not made an error. Blood type AB has both A and B
surface antigens, and neither anti-A nor anti-B antibodies circulating in the
plasma. When anti-A antibodies (added to the first well) contact A antigens on
AB erythrocytes, they will cause agglutination. Similarly, when anti-B
antibodies contact B antigens on AB erythrocytes, they will cause
agglutination.
References
American Red Cross (US). Blood types [Internet]. c2013 [cited 2013 Apr 3].
Glossary
ABO blood group
blood-type classification based on the presence or absence of A and B
glycoproteins on the erythrocyte membrane surface
agglutination
clustering of cells into masses linked by antibodies
cross matching
blood test for identification of blood type using antibodies and small samples
of blood
hemolysis
destruction (lysis) of erythrocytes and the release of their hemoglobin into
circulation
hemolytic disease of the newborn (HDN)
(also, erythroblastosis fetalis) disorder causing agglutination and hemolysis in
an Rh* fetus or newborn of an Rh” mother
Rh blood group
blood-type classification based on the presence or absence of the antigen Rh
on the erythrocyte membrane surface
universal donor
individual with type O" blood
universal recipient
individual with type AB* blood
Introduction to the Cardiovascular System - Heart
class="introduction"
Human Heart
This artist’s
conception
of the human
heart
suggests a
powerful
engine—not
inappropriat
e fora
muscular
pump that
keeps the
body
continually
supplied
with blood.
(credit:
Patrick J.
Lynch)
Note:
Chapter Objectives
After studying this chapter, you will be able to:
e Identify and describe the interior and exterior parts of the human heart
e Describe the path of blood through the cardiac circuits
e Describe the size, shape, and location of the heart
e¢ Compare cardiac muscle to skeletal and smooth muscle
e Explain the cardiac conduction system
e Describe the process and purpose of an electrocardiogram
e Explain the cardiac cycle
e Describe the effects of exercise on heart rate
e Identify other factors affecting heart rate
In this chapter, you will explore the remarkable pump that propels the blood
into the vessels. There is no single better word to describe the function of
the heart other than “pump,” since its contraction develops the pressure that
ejects blood into the major vessels: the aorta and pulmonary trunk. From
these vessels, the blood is distributed to the remainder of the body.
Although the connotation of the term “pump” suggests a mechanical device
made of steel and plastic, the anatomical structure is a living, sophisticated
muscle. As you read this chapter, try to keep these twin concepts in mind:
pump and muscle.
Although the term “heart” is an English word, cardiac (heart-related)
terminology can be traced back to the Latin term, “kardia.” Cardiology is
the study of the heart, and cardiologists are the physicians who deal
primarily with the heart.
Heart Anatomy
By the end of this section, you will be able to:
e Describe the location and position of the heart within the body cavity
e Describe the internal and external anatomy of the heart
e Relate the structure of the heart to its function as a pump
e Compare systemic circulation to pulmonary circulation
e Identify the veins and arteries of the coronary circulation system
e Trace the pathway of oxygenated and deoxygenated blood thorough
the chambers of the heart
The vital importance of the heart is obvious. If one assumes an average rate
of contraction of 75 contractions per minute, a human heart would contract
approximately 108,000 times in one day, more than 39 million times in one
year, and nearly 3 billion times during a 75-year lifespan. Each of the major
pumping chambers of the heart ejects approximately 70 mL blood per
contraction in a resting adult. This would be equal to 5.25 liters of fluid per
minute and approximately 14,000 liters per day. Over one year, that would
equal 10,000,000 liters or 2.6 million gallons of blood sent through roughly
60,000 miles of vessels. In order to understand how that happens, it is
necessary to understand the anatomy and physiology of the heart.
Location of the Heart
The human heart is located within the thoracic cavity and is separated from
the other structures by a tough membrane known as the pericardium, or
pericardial sac.
Position of the Heart in the Thorax
/ Thoracic
Diaphragm r&) aorta
Inferior vena cava
Esophagus
Sagittal view
Mediastinum
; Arch of aorta
Superior vena cava
Right lung Pulmonary trunk
Right auricle Left auricle
Right atrium Left lung
Right ventricle Left ventricle
Ribs (cut) Pericardial cavity
ia
Apex of heart
Diaphragm
Edge of parietal Edge of parietal
pleura (cut) pericardium (cut)
The heart is located within the thoracic cavity, medially
between the lungs in the mediastinum. It is about the size
of a fist, is broad at the top, and tapers toward the base.
Note:
Everyday Connection
CPR
The position of the heart in the torso between the vertebrae and sternum
(see [link] for the position of the heart within the thorax) allows for
individuals to apply an emergency technique known as cardiopulmonary
resuscitation (CPR) if the heart of a patient should stop. By applying
pressure with the flat portion of one hand on the sternum in the area
between the line at T4 and T9 ((link]), it is possible to manually compress
the blood within the heart enough to push some of the blood within it into
the pulmonary and systemic circuits. This is particularly critical for the
brain, as irreversible damage and death of neurons occur within minutes of
loss of blood flow. Current standards call for compression of the chest at
least 5 cm deep and at a rate of 100 compressions per minute, a rate equal
to the beat in “Staying Alive,” recorded in 1977 by the Bee Gees. If you
are unfamiliar with this song, a version is available on www.youtube.com.
At this stage, the emphasis is on performing high-quality chest
compressions, rather than providing artificial respiration. CPR is generally
performed until the patient regains spontaneous contraction or is declared
dead by an experienced healthcare professional.
When performed by untrained or overzealous individuals, CPR can result
in broken ribs or a broken sternum, and can inflict additional severe
damage on the patient. It is also possible, if the hands are placed too low
on the sternum, to manually drive the xiphoid process into the liver, a
consequence that may prove fatal for the patient. Proper training is
essential. This proven life-sustaining technique is so valuable that virtually
all medical personnel as well as concerned members of the public should
be certified and routinely recertified in its application. CPR courses are
offered at a variety of locations, including colleges, hospitals, the
American Red Cross, and some commercial companies. They normally
include practice of the compression technique on a mannequin.
CPR Technique
If the heart should stop, CPR can maintain
the flow of blood until the heart resumes
beating. By applying pressure to the
sternum, the blood within the heart will be
squeezed out of the heart and into the
circulation. Proper positioning of the
hands on the sternum to perform CPR
would be between the lines at T4 and T9.
Shape and Size of the Heart
A typical heart is approximately the size of your fist: 12 cm (5 in) in length,
8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness. Given the size difference
between most members of the sexes, the weight of a female heart is
approximately 250-300 grams (9 to 11 ounces), and the weight of a male
heart is approximately 300-350 grams (11 to 12 ounces). The heart of a
well-trained athlete, especially one specializing in aerobic sports, can be
considerably larger than this. Cardiac muscle responds to exercise in a
manner similar to that of skeletal muscle. That is, exercise results in the
addition of protein myofilaments that increase the size of the individual
cells without increasing their numbers, a concept called hypertrophy. Hearts
of athletes can pump blood more effectively at lower rates than those of
nonathletes. Enlarged hearts are not always a result of exercise; they can
result from diseases, such as hypertrophic cardiomyopathy.
Chambers and Circulation through the Heart
The human heart consists of four chambers: The left side and the right side
each have one atrium and one ventricle. Each of the upper chambers, the
right atrium (plural = atria) and the left atrium, acts as a receiving chamber
and contracts to push blood into the lower chambers, the right ventricle and
the left ventricle. The ventricles serve as the primary pumping chambers of
the heart, propelling blood to the lungs or to the rest of the body.
There are two distinct but linked circuits in the human circulation called the
pulmonary and systemic circuits. Although both circuits transport blood and
everything it carries, we can initially view the circuits from the point of
view of gases. The pulmonary circuit transports blood to and from the
lungs, where it picks up oxygen and delivers carbon dioxide for exhalation.
The systemic circuit transports oxygenated blood to virtually all of the
tissues of the body and returns relatively deoxygenated blood and carbon
dioxide to the heart to be sent back to the pulmonary circulation.
The right ventricle pumps deoxygenated blood into the pulmonary trunk,
which leads toward the lungs and splits into the left and right pulmonary
arteries. These vessels in turn branch many times before reaching the
pulmonary capillaries, where gas exchange occurs: Carbon dioxide exits
the blood and oxygen enters. The pulmonary trunk arteries and their
branches are the only arteries in the post-natal body that carry relatively
deoxygenated blood. Highly oxygenated blood returning from the
pulmonary capillaries in the lungs passes through a series of vessels that
join together to form the pulmonary veins—the only post-natal veins in the
body that carry highly oxygenated blood. The pulmonary veins conduct
blood into the left atrium, which pumps the blood into the left ventricle,
which in turn pumps oxygenated blood into the aorta and on to the many
branches of the systemic circuit. Eventually, these vessels will lead to the
systemic capillaries, where exchange with the tissue fluid and cells of the
body occurs. In this case, oxygen and nutrients exit the systemic capillaries
to be used by the cells in their metabolic processes, and carbon dioxide and
waste products will enter the blood.
The blood exiting the systemic capillaries is lower in oxygen concentration
than when it entered. The capillaries will ultimately unite to form venules,
joining to form ever-larger veins, eventually flowing into the two major
systemic veins, the superior vena cava and the inferior vena cava, which
return blood to the right atrium. The blood in the superior and inferior
venae cavae flows into the right atrium, which pumps blood into the right
ventricle. This process of blood circulation continues as long as the
individual remains alive. Understanding the flow of blood through the
pulmonary and systemic circuits is critical to all health professions ({link]).
Dual System of the Human Blood Circulation
Aorta
Left pulmonary
Right pulmonary arteries
arteries
Pulmonary trunk
Left atrium
Right pulmonary
veins Left pulmonary
veins
Pulmonary
semilunar
valve
Aortic semilunar
valve
Mitral valve
Right atrium
Tricuspid valve Left ventricle
Right ventricle
Systemic Systemic
veins from capillaries
upper body of upper
body
Systemic
arteries to
Pulmona
es upper body
capillaries
in lungs
Pulmonary
Right atrium trunk
Left atrium
Right
ventricle Left
\ ventricle
Systemic
veins from
Systemic
lower body arteries to
lower body
Systemic
capillaries
of lower body
Blood flows from the right atrium to the right ventricle,
where it is pumped into the pulmonary circuit. The blood in
the pulmonary artery branches is low in oxygen but
relatively high in carbon dioxide. Gas exchange occurs in
the pulmonary capillaries (oxygen into the blood, carbon
dioxide out), and blood high in oxygen and low in carbon
dioxide is returned to the left atrium. From here, blood
enters the left ventricle, which pumps it into the systemic
circuit. Following exchange in the systemic capillaries
(oxygen and nutrients out of the capillaries and carbon
dioxide and wastes in), blood returns to the right atrium and
the cycle is repeated.
Membranes, Surface Features, and Layers
Layers
The wall of the heart is composed of three layers of unequal thickness.
From superficial to deep, these are the epicardium, the myocardium, and the
endocardium. The outermost layer of the wall of the heart is also the
innermost layer of the pericardium, the epicardium, or the visceral
pericardium discussed earlier.
The middle and thickest layer is the myocardium, made largely of cardiac
muscle cells. It is built upon a framework of collagenous fibers, plus the
blood vessels that supply the myocardium and the nerve fibers that help
regulate the heart. It is the contraction of the myocardium that pumps blood
through the heart and into the major arteries. The muscle pattern is elegant
and complex, as the muscle cells swirl and spiral around the chambers of
the heart. They form a figure 8 pattern around the atria and around the bases
of the great vessels. Deeper ventricular muscles also form a figure 8 around
the two ventricles and proceed toward the apex. More superficial layers of
ventricular muscle wrap around both ventricles. This complex swirling
pattern allows the heart to pump blood more effectively than a simple linear
pattern would. [link] illustrates the arrangement of muscle cells.
Heart Musculature
Atrial
musculature
Ventricular
musculature
The swirling pattern of
cardiac muscle tissue
contributes significantly to
the heart’s ability to pump
blood effectively.
Although the ventricles on the right and left sides pump the same amount of
blood per contraction, the muscle of the left ventricle is much thicker and
better developed than that of the right ventricle. In order to overcome the
high resistance required to pump blood into the long systemic circuit, the
left ventricle must generate a great amount of pressure. The right ventricle
does not need to generate as much pressure, since the pulmonary circuit is
shorter and provides less resistance. [link] illustrates the differences in
muscular thickness needed for each of the ventricles.
Differences in Ventricular Muscle Thickness
Left ventricle
Right ventricle
Relaxed Contracted
The myocardium in the left ventricle is significantly
thicker than that of the right ventricle. Both ventricles
pump the same amount of blood, but the left ventricle
must generate a much greater pressure to overcome
greater resistance in the systemic circuit. The
ventricles are shown in both relaxed and contracting
states. Note the differences in the relative size of the
lumens, the region inside each ventricle where the
blood is contained.
Internal Structure of the Heart
Recall that the heart’s contraction cycle follows a dual pattern of circulation
—the pulmonary and systemic circuits—because of the pairs of chambers
that pump blood into the circulation. In order to develop a more precise
understanding of cardiac function, it is first necessary to explore the internal
anatomical structures in more detail.
Septa of the Heart
There are four openings that allow blood to move from the atria into the
ventricles and from the ventricles into the pulmonary trunk and aorta.
Located in each of these openings between the atria and ventricles is a
valve, a specialized structure that ensures one-way flow of blood. The
valves between the atria and ventricles are known generically as
atrioventricular (AV) valves. The valves at the openings that lead to the
pulmonary trunk and aorta are known generically as semilunar valves.
Internal Structures of the Heart
Aorta
Superior vena cava Left pulmonary artery
Right pulmonary artery Left atrium
Pulmonary trunk Left pulmonary veins
Right pulmonary
veins Mitral (bicuspid) valve
Right atrium
. Aortic valve
Fossa ovalis
Tricuspid valve Pulmonary valve
Right ventricle Left ventricle
Chordae tendineae Papillary muscle
———— Interventricular septum
Epicardium
Myocardium
Endocardium
Trabeculae carneae
Moderator band
Inferior vena cava
Anterior view
This anterior view of the heart shows the four
chambers, the major vessels and their early branches,
as well as the valves.
Right Atrium
The right atrium serves as the receiving chamber for blood returning to the
heart from the systemic circulation. The two major systemic veins, the
superior and inferior venae cavae, and the large coronary vein called the
coronary sinus that drains the heart myocardium empty into the right
atrium. The superior vena cava drains blood from regions abover the
diaphragm: the head, neck, upper limbs, and the thoracic region. It empties
into the superior and posterior portions of the right atrium. The inferior
vena cava drains blood from areas below the diaphragm: the lower limbs
and abdominal and pelvic region of the body. It, too, empties into the
posterior portion of the atria, but is below the opening of the superior vena
cava. The majority of the internal heart structures discussed in this and
subsequent sections are illustrated in the provided figure.
The atria receive venous blood on a nearly continuous basis, preventing
venous flow from stopping while the ventricles are contracting. While most
ventricular filling occurs while the atria are relaxed, they do demonstrate a
contractile phase and actively pump blood into the ventricles just prior to
ventricular contraction. The opening between the atrium and ventricle is
guarded by the tricuspid valve.
Right Ventricle
The right ventricle receives blood from the right atrium through the
tricuspid valve. Each flap of the valve is attached to strong strands of
connective tissue, the chordae tendineae, literally “tendinous cords,” or
sometimes more poetically referred to as “heart strings.” There are several
chordae tendineae associated with each of the flaps. They are composed of
approximately 80 percent collagenous fibers with the remainder consisting
of elastic fibers and endothelium. They connect each of the flaps to a
papillary muscle.
When the myocardium of the ventricle contracts, pressure within the
ventricular chamber rises. Blood, like any fluid, flows from higher pressure
to lower pressure areas, in this case, toward the pulmonary trunk and the
atrium. To prevent any potential backflow, the papillary muscles also
contract, generating tension on the chordae tendineae. This prevents the
flaps of the valves from being forced into the atria and regurgitation of the
blood back into the atria during ventricular contraction. [link] shows
papillary muscles and chordae tendineae attached to the tricuspid valve.
Chordae Tendineae and Papillary Muscles
Chordae tendineae
Papillary muscles
Trabeculae carneae
In this frontal section, you can see papillary
muscles attached to the tricuspid valve on
the right as well as the mitral valve on the
left via chordae tendineae. (credit:
modification of work by “PV
KS”/flickr.com)
When the right ventricle contracts, it ejects blood into the pulmonary trunk,
which branches into the left and right pulmonary arteries that carry it to
each lung. The superior surface of the right ventricle begins to taper as it
approaches the pulmonary trunk. At the base of the pulmonary trunk is the
pulmonary semilunar valve that prevents backflow from the pulmonary
trunk.
Left Atrium
After exchange of gases in the pulmonary capillaries, blood returns to the
left atrium high in oxygen via one of the four pulmonary veins. Blood flows
nearly continuously from the pulmonary veins back into the atrium, which
acts as the receiving chamber, and from here through an opening into the
left ventricle. Most blood flows passively into the heart while both the atria
and ventricles are relaxed, but toward the end of the ventricular relaxation
period, the left atrium will contract, pumping blood into the ventricle. This
atrial contraction accounts for approximately 20 percent of ventricular
filling. The opening between the left atrium and ventricle is guarded by the
mitral valve.
Left Ventricle
Recall that, although both sides of the heart will pump the same amount of
blood, the muscular layer is much thicker in the left ventricle compared to
the right (see [link]). The mitral valve is connected to papillary muscles via
chordae tendineae.
The left ventricle is the major pumping chamber for the systemic circuit; it
ejects blood into the aorta through the aortic semilunar valve.
Heart Valve Structure and Function
A transverse section through the heart slightly above the level of the
atrioventricular septum reveals all four heart valves along the same plane
({link]). The valves ensure unidirectional blood flow through the heart.
Between the right atrium and the right ventricle is the right
atrioventricular valve, or tricuspid valve. It typically consists of three
flaps, or leaflets, made of endocardium reinforced with additional
connective tissue. The flaps are connected by chordae tendineae to the
papillary muscles, which control the opening and closing of the valves.
Heart Valves
Posterior
Tricuspid valve Bicuspid (mitral)
Right
side of
heart
Aortic valve Pulmonary valve
Anterior
With the atria and major vessels removed, all
four valves are clearly visible, although it is
difficult to distinguish the three separate cusps
of the tricuspid valve.
Emerging from the right ventricle at the base of the pulmonary trunk is the
pulmonary semilunar valve or the right semilunar valve. The pulmonary
valve is comprised of three small flaps of endothelium reinforced with
connective tissue. When the ventricle relaxes, the pressure differential
causes blood to flow back into the ventricle from the pulmonary trunk. This
flow of blood fills the pocket-like flaps of the pulmonary valve, causing the
valve to close and producing an audible sound ("Lub"). Unlike the
atrioventricular valves, there are no papillary muscles or chordae tendineae
associated with the pulmonary valve.
Located at the opening between the left atrium and left ventricle is the
mitral valve, also called the bicuspid valve or the left atrioventricular
valve. Structurally, this valve consists of two cusps compared to the three
cusps of the tricuspid valve. In a clinical setting, the valve is referred to as
the mitral valve, rather than the bicuspid valve. The two cusps of the mitral
valve are attached by chordae tendineae to two papillary muscles that
project from the wall of the ventricle.
At the base of the aorta is the aortic semilunar valve, or the aortic valve,
which prevents backflow from the aorta. It normally is composed of three
flaps. When the ventricle relaxes and blood attempts to flow back into the
ventricle from the aorta, blood will fill the cusps of the valve, causing it to
close and producing an audible sound ("dub/dup").
In [link]a, the two atrioventricular valves are open and the two semilunar
valves are closed. This occurs when both atria and ventricles are relaxed
and when the atria contract to pump blood into the ventricles. [link]b shows
a frontal view. Although only the left side of the heart is illustrated, the
process is virtually identical on the right.
Blood Flow from the Left Atrium to the Left Ventricle
Posterior
Tricuspid
valve
(open)
<
Bicuspid (mitral) valve
(closed)
Anterior Aploned)
(a) Mitral
valve
(open)
Aortic valve
(Closed)
Chordae
tendineae
(loose)
Papillary
muscle
(relaxed)
(b)
(a) A transverse section through the heart illustrates
the four heart valves. The two atrioventricular valves
are open; the two semilunar valves are closed. The
atria and vessels have been removed. (b) A frontal
section through the heart illustrates blood flow
through the mitral valve. When the mitral valve is
open, it allows blood to move from the left atrium to
the left ventricle. The aortic semilunar valve is closed
to prevent backflow of blood from the aorta to the left
ventricle.
[link]a shows the atrioventricular valves closed while the two semilunar
valves are open. This occurs when the ventricles contract to eject blood into
the pulmonary trunk and aorta. Closure of the two atrioventricular valves
prevents blood from being forced back into the atria. This stage can be seen
from a frontal view in [link]b.
Blood Flow from the Left Ventricle into the Great Vessels
Posterior
Tricuspid Bicuspid (mitral) valve
valve —— ~ £ (closed)
(closed) “a . A
Left
side of \\\ . \ side of
heart
Aortic
valve
(open)
Pulmonary
valve
Anterior open)
(a) Mitral
valve
(closed)
Aortic valve
(open)
Chordae
tendineae
(tight)
Papillary
muscle
(contracted)
(b)
(a) A transverse section through the heart illustrates
the four heart valves during ventricular contraction.
The two atrioventricular valves are closed, but the two
semilunar valves are open. The atria and vessels have
been removed. (b) A frontal view shows the closed
mitral (bicuspid) valve that prevents backflow of
blood into the left atrium. The aortic semilunar valve
is open to allow blood to be ejected into the aorta.
When the ventricles begin to contract, pressure within the ventricles rises
and blood flows toward the area of lowest pressure, which is initially in the
atria. This backflow causes the cusps of the tricuspid and mitral (bicuspid)
valves to close. These valves are tied down to the papillary muscles by
chordae tendineae. During the relaxation phase of the cardiac cycle, the
papillary muscles are also relaxed and the tension on the chordae tendineae
is slight (see [link]b). However, as the myocardium of the ventricle
contracts, so do the papillary muscles. This creates tension on the chordae
tendineae (see [link]b), helping to hold the cusps of the atrioventricular
valves in place and preventing them from being blown back into the atria.
The aortic and pulmonary semilunar valves lack the chordae tendineae and
papillary muscles associated with the atrioventricular valves. Instead, they
consist of pocket-like folds of endocardium reinforced with additional
connective tissue. When the ventricles relax and the change in pressure
forces the blood toward the ventricles, the blood presses against these cusps
and seals the openings.
Note:
meme OPENStAX COLLEGE”
—
F
coe
Visit this site to observe an echocardiogram of actual heart valves opening
and closing. Although much of the heart has been “removed” from this gif
loop so the chordae tendineae are not visible, why is their presence more
critical for the atrioventricular valves (tricuspid and mitral) than the
semilunar (aortic and pulmonary) valves?
Note:
Disorders of the...
Heart Valves
When heart valves do not function properly, they are often described as
incompetent and result in valvular heart disease, which can range from
benign to lethal. Some of these conditions are congenital, that is, the
individual was born with the defect, whereas others may be attributed to
disease processes or trauma. Some malfunctions are treated with
medications, others require surgery, and still others may be mild enough
that the condition is merely monitored since treatment might trigger more
Serious Consequences.
Valvular disorders are often caused by carditis, or inflammation of the
heart. One common trigger for this inflammation is rheumatic fever, or
scarlet fever, an autoimmune response to the presence of a bacterium,
Streptococcus pyogenes, normally a disease of childhood.
While any of the heart valves may be involved in valve disorders, mitral
regurgitation is the most common, detected in approximately 2 percent of
the population, and the pulmonary semilunar valve is the least frequently
involved. When a valve malfunctions, the flow of blood to a region will
often be disrupted. The resulting inadequate flow of blood to this region
will be described in general terms as an insufficiency. The specific type of
insufficiency is named for the valve involved: aortic insufficiency, mitral
insufficiency, tricuspid insufficiency, or pulmonary insufficiency.
If one of the cusps of the valve is forced backward by the force of the
blood, the condition is referred to as a prolapsed valve. Prolapse may occur
if the chordae tendineae are damaged or broken, causing the closure
mechanism to fail. The failure of the valve to close properly disrupts the
normal one-way flow of blood and results in regurgitation, when the blood
flows backward from its normal path. Using a stethoscope, the disruption
to the normal flow of blood produces a heart murmur.
Stenosis is a condition in which the heart valves become rigid and may
calcify over time. The loss of flexibility of the valve interferes with normal
function and may cause the heart to work harder to propel blood through
the valve, which eventually weakens the heart. Aortic stenosis affects
approximately 2 percent of the population over 65 years of age, and the
percentage increases to approximately 4 percent in individuals over 85
years. Occasionally, one or more of the chordae tendineae will tear or the
papillary muscle itself may die as a component of a myocardial infarction
(heart attack). In this case, the patient’s condition will deteriorate
dramatically and rapidly, and immediate surgical intervention may be
required.
Auscultation, or listening to a patient’s heart sounds, is one of the most
useful diagnostic tools, since it is proven, safe, and inexpensive. The term
auscultation is derived from the Latin for “to listen,” and the technique has
been used for diagnostic purposes as far back as the ancient Egyptians.
Valve and septal disorders will trigger abnormal heart sounds. If a valvular
disorder is detected or suspected, a test called an echocardiogram, or
simply an “echo,” may be ordered. Echocardiograms are sonograms of the
heart and can help in the diagnosis of valve disorders as well as a wide
variety of heart pathologies.
Note:
[= aC
Visit this site for a free download, including excellent animations and
audio of heart sounds.
Note:
Career Connection
Cardiologist
Cardiologists are medical doctors that specialize in the diagnosis and
treatment of diseases of the heart. After completing 4 years of medical
school, cardiologists complete a three-year residency in internal medicine
followed by an additional three or more years in cardiology. Following this
10-year period of medical training and clinical experience, they qualify for
a rigorous two-day examination administered by the Board of Internal
Medicine that tests their academic training and clinical abilities, including
diagnostics and treatment. After successful completion of this examination,
a physician becomes a board-certified cardiologist. Some board-certified
cardiologists may be invited to become a Fellow of the American College
of Cardiology (FACC). This professional recognition is awarded to
outstanding physicians based upon merit, including outstanding
credentials, achievements, and community contributions to cardiovascular
medicine.
Note:
uae
—_ 7
= openstax COLLEGE”
Visit this site to learn more about cardiologists.
Note:
Career Connection
Cardiovascular Technologist/Technician
Cardiovascular technologists/technicians are trained professionals who
perform a variety of imaging techniques, such as sonograms or
echocardiograms, used by physicians to diagnose and treat diseases of the
heart. Nearly all of these positions require an associate degree, and these
technicians earn a median salary of $49,410 as of May 2010, according to
the U.S. Bureau of Labor Statistics. Growth within the field is fast,
projected at 29 percent from 2010 to 2020.
There is a considerable overlap and complementary skills between cardiac
technicians and vascular technicians, and so the term cardiovascular
technician is often used. Special certifications within the field require
documenting appropriate experience and completing additional and often
expensive certification examinations. These subspecialties include
Certified Rhythm Analysis Technician (CRAT), Certified Cardiographic
Technician (CCT), Registered Congenital Cardiac Sonographer (RCCS),
Registered Cardiac Electrophysiology Specialist (RCES), Registered
Cardiovascular Invasive Specialist (RCIS), Registered Cardiac
Sonographer (RCS), Registered Vascular Specialist (RVS), and Registered
Phlebology Sonographer (RPhS).
Note:
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=> openstax COLLEGE”
ine t
[yr
Visit this site for more information on cardiovascular
technologists/technicians.
Coronary Circulation
You will recall that the heart is a remarkable pump composed largely of
cardiac muscle cells that are incredibly active throughout life. Like all other
cells, a cardiomyocyte requires a reliable supply of oxygen and nutrients,
and a way to remove wastes, so it needs a dedicated, complex, and
extensive coronary circulation. And because of the critical and nearly
ceaseless activity of the heart throughout life, this need for a blood supply is
even greater than for a typical cell. However, coronary circulation is not
continuous; rather, it cycles, reaching a peak when the heart muscle is
relaxed and nearly ceasing while it is contracting.
Coronary Arteries and Veins
Coronary arteries supply blood to the myocardium and other components
of the heart. The first portion of the aorta after it arises from the left
ventricle gives rise to the coronary arteries (right and left), which bring
freshly oxygenated blood to the tissues of both sides of the heart. A
coronary artery blockage often results in death of the cells (due to
myocardial infarction/heart attack) supplied by the particular blood vessel.
Coronary veins drain the heart and generally parallel the large surface
arteries.
Note:
Diseases of the...
Heart: Myocardial Infarction
Myocardial infarction (MI) is the formal term for what is commonly
referred to as a heart attack. It normally results from a lack of blood flow
(ischemia) and oxygen (hypoxia) to a region of the heart, resulting in death
of the cardiac muscle cells. An MI often occurs when a coronary artery is
blocked by the buildup of atherosclerotic plaque consisting of lipids,
cholesterol and fatty acids, and white blood cells, primarily macrophages.
It can also occur when a portion of an unstable atherosclerotic plaque
travels through the coronary arterial system and lodges in one of the
smaller vessels. The resulting blockage restricts the flow of blood and
oxygen to the myocardium and causes death of the tissue. MIs may be
triggered by excessive exercise, in which the partially occluded artery is no
longer able to pump sufficient quantities of blood, or severe stress, which
may induce spasm of the smooth muscle in the walls of the vessel.
In the case of acute MI, there is often sudden pain beneath the sternum
called angina pectoris, often radiating down the left arm in males but not in
female patients. Until this anomaly between the sexes was discovered,
many female patients suffering MIs were misdiagnosed and sent home. In
addition, patients typically present with difficulty breathing and shortness
of breath, irregular heartbeat, nausea and vomiting, sweating, anxiety, and
fainting, although not all of these symptoms may be present. Many of the
symptoms are shared with other medical conditions, including anxiety
attacks and simple indigestion, so differential diagnosis is critical. It is
estimated that between 22 and 64 percent of MIs present without any
symptoms.
Immediate treatments for MI are essential and include administering
supplemental oxygen, aspirin that helps to break up clots, and
nitroglycerine administered sublingually (under the tongue) to facilitate its
absorption. Despite its unquestioned success in treatments and use since
the 1880s, the mechanism of nitroglycerine is still incompletely understood
but is believed to involve the release of nitric oxide, a known vasodilator,
and endothelium-derived releasing factor, which also relaxes the smooth
muscle in the tunica media of coronary vessels. Longer-term treatments
include injections of thrombolytic agents such as streptokinase that
dissolve the clot, the anticoagulant heparin, balloon angioplasty and stents
to open blocked vessels, and bypass surgery to allow blood to pass around
the site of blockage. If the damage is extensive, coronary replacement with
a donor heart or coronary assist device, a sophisticated mechanical device
that supplements the pumping activity of the heart, may be employed.
Despite the attention, development of artificial hearts to augment the
severely limited supply of heart donors has proven less than satisfactory
but will likely improve in the future.
Coronary Artery Disease
Note:
Coronary artery disease is the leading cause of death worldwide. It occurs
when the buildup of plaque—a fatty material including cholesterol,
connective tissue, white blood cells, and some smooth muscle cells—
within the walls of the arteries obstructs the flow of blood and decreases
the flexibility or compliance of the vessels. This condition is called
atherosclerosis, a hardening of the arteries that involves the accumulation
of plaque. As the coronary blood vessels become occluded, the flow of
blood to the tissues will be restricted, a condition called ischemia that
causes the cells to receive insufficient amounts of oxygen, called hypoxia.
[link] shows the blockage of coronary arteries highlighted by the injection
of dye. Some individuals with coronary artery disease report pain radiating
from the chest called angina pectoris, but others remain asymptomatic. If
untreated, coronary artery disease can lead to MI or a heart attack.
Atherosclerotic Coronary Arteries
> ~
Blockage of common trunk “wy
of left coronary artery
In this coronary angiogram (X-ray), the dye
makes visible two occluded coronary
arteries. Such blockages can lead to
decreased blood flow (ischemia) and
insufficient oxygen (hypoxia) delivered to
the cardiac tissues. If uncorrected, this can
lead to cardiac muscle death (myocardial
infarction).
The disease progresses slowly and often begins in children and can be seen
as fatty “streaks” in the vessels. It then gradually progresses throughout
life. Well-documented risk factors include smoking, family history,
hypertension, obesity, diabetes, high alcohol consumption, lack of exercise,
stress, and hyperlipidemia or high circulating levels of lipids in the blood.
Treatments may include medication, changes to diet and exercise,
angioplasty with a balloon catheter, insertion of a stent, or coronary bypass
procedure.
Angioplasty is a procedure in which the occlusion is mechanically widened
with a balloon. A specialized catheter with an expandable tip is inserted
into a superficial vessel, normally in the leg, and then directed to the site of
the occlusion. At this point, the balloon is inflated to compress the plaque
material and to open the vessel to increase blood flow. Then, the balloon is
deflated and retracted. A stent consisting of a specialized mesh is typically
inserted at the site of occlusion to reinforce the weakened and damaged
walls. Stent insertions have been routine in cardiology for more than 40
years.
Coronary bypass surgery may also be performed. This surgical procedure
grafts a replacement vessel obtained from another, less vital portion of the
body to bypass the occluded area. This procedure is clearly effective in
treating patients experiencing a MI, but overall does not increase longevity.
Nor does it seem advisable in patients with stable although diminished
cardiac capacity since frequently loss of mental acuity occurs following the
procedure. Long-term changes to behavior, emphasizing diet and exercise
plus a medicine regime tailored to lower blood pressure, lower cholesterol
and lipids, and reduce clotting are equally as effective.
Chapter Review
The heart resides within the pericardial sac. The walls of the heart are
composed of three layers of tissue, including a thick myocardium. The
human heart consists of a pair of atria, which receive blood and pump it into
a pair of ventricles, which pump blood into the vessels. The right atrium
receives systemic blood relatively low in oxygen and pumps it into the right
ventricle, which pumps it into the pulmonary circuit. Exchange of oxygen
and carbon dioxide occurs in the lungs, and blood high in oxygen returns to
the left atrium, which pumps blood into the left ventricle, which in turn
pumps blood into the aorta and the remainder of the systemic circuit. The
two openings between the atria and ventricles are guarded by the
atrioventricular valves, the right tricuspid valve and the left mitral valve,
which prevent the backflow of blood when the ventricles contract. Each is
attached to chordae tendineae that extend to the papillary muscles, which
are extensions of the myocardium, to prevent the valves from being blown
back into the atria. The pulmonary semilunar valve is located at the base of
the pulmonary trunk, and the left semilunar valve is located at the base of
the aorta. The right and left coronary arteries are the first to branch off the
aorta. Cardiac veins parallel the cardiac arteries and eventually drain into
the right atrium.
Review Questions
Exercise:
Problem:
Which of the following is not important in preventing backflow of
blood?
a. chordae tendineae
b. papillary muscles
c. AV valves
d. endocardium
Solution:
D
Exercise:
Problem: Which valve separates the left atrium from the left ventricle?
a. mitral
b. tricuspid
c. pulmonary
d. aortic
Solution:
A
Exercise:
Problem:
Which of the following lists the valves in the order through which the
blood flows from the vena cava through the heart?
a. tricuspid, pulmonary semilunar, bicuspid, aortic semilunar
b. mitral, pulmonary semilunar, bicuspid, aortic semilunar
c. aortic semilunar, pulmonary semilunar, tricuspid, bicuspid
d. bicuspid, aortic semilunar, tricuspid, pulmonary semilunar
Solution:
A
Exercise:
Problem:
Which chamber initially receives blood from the systemic circuit?
a. left atrium
b. left ventricle
c. right atrium
d. right ventricle
Solution:
C
Exercise:
Problem:The myocardium would be the thickest in the
a. left atrium
b. left ventricle
c. right atrium
d. right ventricle
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Describe how the valves keep the blood moving in one direction.
Solution:
When the ventricles contract and pressure begins to rise in the
ventricles, there is an initial tendency for blood to flow back
(regurgitate) to the atria. However, the papillary muscles also contract,
placing tension on the chordae tendineae and holding the
atrioventricular valves (tricuspid and mitral) in place to prevent the
valves from prolapsing and being forced back into the atria. The
semilunar valves (pulmonary and aortic) lack chordae tendineae and
papillary muscles, but do not face the same pressure gradients as do
the atrioventricular valves. As the ventricles relax and pressure drops
within the ventricles, there is a tendency for the blood to flow
backward. However, the valves, consisting of reinforced endothelium
and connective tissue, fill with blood and seal off the opening
preventing the return of blood.
Exercise:
Problem:
Why is the pressure in the pulmonary circulation lower than in the
systemic circulation?
Solution:
The pulmonary circuit consists of blood flowing to and from the lungs,
whereas the systemic circuit carries blood to and from the entire body.
The systemic circuit is far more extensive, consisting of far more
vessels and offers much greater resistance to the flow of blood, so the
heart must generate a higher pressure to overcome this resistance. This
can be seen in the thickness of the myocardium in the ventricles.
Glossary
anastomosis
(plural = anastomoses) area where vessels unite to allow blood to
circulate even if there may be partial blockage in another branch
anterior cardiac veins
vessels that parallel the small cardiac arteries and drain the anterior
surface of the right ventricle; bypass the coronary sinus and drain
directly into the right atrium
anterior interventricular artery
(also, left anterior descending artery or LAD) major branch of the left
coronary artery that follows the anterior interventricular sulcus
anterior interventricular sulcus
sulcus located between the left and right ventricles on the anterior
surface of the heart
aortic valve
(also, aortic semilunar valve) valve located at the base of the aorta
atrioventricular septum
cardiac septum located between the atria and ventricles;
atrioventricular valves are located here
atrioventricular valves
one-way valves located between the atria and ventricles; the valve on
the right is called the tricuspid valve, and the one on the left is the
mitral or bicuspid valve
atrium
(plural = atria) upper or receiving chamber of the heart that pumps
blood into the lower chambers just prior to their contraction; the right
atrium receives blood from the systemic circuit that flows into the right
ventricle; the left atrium receives blood from the pulmonary circuit
that flows into the left ventricle
auricle
extension of an atrium visible on the superior surface of the heart
bicuspid valve
(also, mitral valve or left atrioventricular valve) valve located between
the left atrium and ventricle; consists of two flaps of tissue
cardiac notch
depression in the medial surface of the inferior lobe of the left lung
where the apex of the heart is located
cardiac skeleton
(also, skeleton of the heart) reinforced connective tissue located within
the atrioventricular septum; includes four rings that surround the
openings between the atria and ventricles, and the openings to the
pulmonary trunk and aorta; the point of attachment for the heart valves
cardiomyocyte
muscle cell of the heart
chordae tendineae
string-like extensions of tough connective tissue that extend from the
flaps of the atrioventricular valves to the papillary muscles
circumflex artery
branch of the left coronary artery that follows coronary sulcus
coronary arteries
branches of the ascending aorta that supply blood to the heart; the left
coronary artery feeds the left side of the heart, the left atrium and
ventricle, and the interventricular septum; the right coronary artery
feeds the right atrium, portions of both ventricles, and the heart
conduction system
coronary sinus
large, thin-walled vein on the posterior surface of the heart that lies
within the atrioventricular sulcus and drains the heart myocardium
directly into the right atrium
coronary sulcus
sulcus that marks the boundary between the atria and ventricles
coronary veins
vessels that drain the heart and generally parallel the large surface
arteries
endocardium
innermost layer of the heart lining the heart chambers and heart valves;
composed of endothelium reinforced with a thin layer of connective
tissue that binds to the myocardium
endothelium
layer of smooth, simple squamous epithelium that lines the
endocardium and blood vessels
epicardial coronary arteries
surface arteries of the heart that generally follow the sulci
epicardium
innermost layer of the serous pericardium and the outermost layer of
the heart wall
foramen ovale
opening in the fetal heart that allows blood to flow directly from the
right atrium to the left atrium, bypassing the fetal pulmonary circuit
fossa ovalis
oval-shaped depression in the interatrial septum that marks the former
location of the foramen ovale
great cardiac vein
vessel that follows the interventricular sulcus on the anterior surface of
the heart and flows along the coronary sulcus into the coronary sinus
on the posterior surface; parallels the anterior interventricular artery
and drains the areas supplied by this vessel
hypertrophic cardiomyopathy
pathological enlargement of the heart, generally for no known reason
inferior vena cava
large systemic vein that returns blood to the heart from the inferior
portion of the body
interatrial septum
cardiac septum located between the two atria; contains the fossa ovalis
after birth
interventricular septum
cardiac septum located between the two ventricles
left atrioventricular valve
(also, mitral valve or bicuspid valve) valve located between the left
atrium and ventricle; consists of two flaps of tissue
marginal arteries
branches of the right coronary artery that supply blood to the
superficial portions of the right ventricle
mesothelium
simple squamous epithelial portion of serous membranes, such as the
superficial portion of the epicardium (the visceral pericardium) and the
deepest portion of the pericardium (the parietal pericardium)
middle cardiac vein
vessel that parallels and drains the areas supplied by the posterior
interventricular artery; drains into the great cardiac vein
mitral valve
(also, left atrioventricular valve or bicuspid valve) valve located
between the left atrium and ventricle; consists of two flaps of tissue
moderator band
band of myocardium covered by endocardium that arises from the
inferior portion of the interventricular septum in the right ventricle and
crosses to the anterior papillary muscle; contains conductile fibers that
carry electrical signals followed by contraction of the heart
myocardium
thickest layer of the heart composed of cardiac muscle cells built upon
a framework of primarily collagenous fibers and blood vessels that
supply it and the nervous fibers that help to regulate it
papillary muscle
extension of the myocardium in the ventricles to which the chordae
tendineae attach
pectinate muscles
muscular ridges seen on the anterior surface of the right atrium
pericardial cavity
cavity surrounding the heart filled with a lubricating serous fluid that
reduces friction as the heart contracts
pericardial sac
(also, pericardium) membrane that separates the heart from other
mediastinal structures; consists of two distinct, fused sublayers: the
fibrous pericardium and the parietal pericardium
pericardium
(also, pericardial sac) membrane that separates the heart from other
mediastinal structures; consists of two distinct, fused sublayers: the
fibrous pericardium and the parietal pericardium
posterior cardiac vein
vessel that parallels and drains the areas supplied by the marginal
artery branch of the circumflex artery; drains into the great cardiac
vein
posterior interventricular artery
(also, posterior descending artery) branch of the right coronary artery
that runs along the posterior portion of the interventricular sulcus
toward the apex of the heart and gives rise to branches that supply the
interventricular septum and portions of both ventricles
posterior interventricular sulcus
sulcus located between the left and right ventricles on the anterior
surface of the heart
pulmonary arteries
left and right branches of the pulmonary trunk that carry deoxygenated
blood from the heart to each of the lungs
pulmonary capillaries
capillaries surrounding the alveoli of the lungs where gas exchange
occurs: carbon dioxide exits the blood and oxygen enters
pulmonary circuit
blood flow to and from the lungs
pulmonary trunk
large arterial vessel that carries blood ejected from the right ventricle;
divides into the left and right pulmonary arteries
pulmonary valve
(also, pulmonary semilunar valve, the pulmonic valve, or the right
semilunar valve) valve at the base of the pulmonary trunk that prevents
backflow of blood into the right ventricle; consists of three flaps
pulmonary veins
veins that carry highly oxygenated blood into the left atrium, which
pumps the blood into the left ventricle, which in turn pumps
oxygenated blood into the aorta and to the many branches of the
systemic circuit
right atrioventricular valve
(also, tricuspid valve) valve located between the right atrium and
ventricle; consists of three flaps of tissue
semilunar valves
valves located at the base of the pulmonary trunk and at the base of the
aorta
septum
(plural = septa) walls or partitions that divide the heart into chambers
septum primum
flap of tissue in the fetus that covers the foramen ovale within a few
seconds after birth
small cardiac vein
parallels the right coronary artery and drains blood from the posterior
surfaces of the right atrium and ventricle; drains into the great cardiac
vein
sulcus
(plural = sulci) fat-filled groove visible on the surface of the heart;
coronary vessels are also located in these areas
superior vena cava
large systemic vein that returns blood to the heart from the superior
portion of the body
systemic circuit
blood flow to and from virtually all of the tissues of the body
trabeculae carneae
ridges of muscle covered by endocardium located in the ventricles
tricuspid valve
term used most often in clinical settings for the right atrioventricular
valve
valve
in the cardiovascular system, a specialized structure located within the
heart or vessels that ensures one-way flow of blood
ventricle
one of the primary pumping chambers of the heart located in the lower
portion of the heart; the left ventricle is the major pumping chamber on
the lower left side of the heart that ejects blood into the systemic
circuit via the aorta and receives blood from the left atrium; the right
ventricle is the major pumping chamber on the lower right side of the
heart that ejects blood into the pulmonary circuit via the pulmonary
trunk and receives blood from the right atrium
Cardiac Muscle and Electrical Activity
By the end of this section, you will be able to:
e Describe the structure of cardiac muscle
¢ Identify and describe the components of the conducting system that
distributes electrical impulses through the heart
e Relate characteristics of an electrocardiogram to events in the cardiac
cycle
¢ Identify blocks that can interrupt the cardiac cycle
Recall that cardiac muscle shares a few characteristics with both skeletal
muscle and smooth muscle, but it has some unique properties of its own.
Not the least of these exceptional properties is its ability to initiate an
electrical potential at a fixed rate that spreads rapidly from cell to cell to
trigger the contraction mechanism. This property is known as
autorhythmicity. Neither smooth nor skeletal muscle can do this. Even
though cardiac muscle has autorhythmicity, heart rate is modulated by the
endocrine and nervous systems.
Structure of Cardiac Muscle
Compared to the giant cylinders of skeletal muscle, cardiac muscle cells, or
cardiomyocytes, are considerably shorter with much smaller diameters.
Cardiac muscle also demonstrates striations, the alternating pattern of dark
A bands and light I bands attributed to the precise arrangement of the
myofilaments and fibrils that are organized in sarcomeres along the length
of the cell ({link]a). These contractile elements are virtually identical to
skeletal muscle. T (transverse) tubules penetrate from the surface plasma
membrane, the sarcolemma, to the interior of the cell, allowing the
electrical impulse to reach the interior. The T tubules are only found at the
Z, discs, whereas in skeletal muscle, they are found at the junction of the A
and I bands. Therefore, there are one-half as many T tubules in cardiac
muscle as in skeletal muscle. In addition, the sarcoplasmic reticulum stores
few calcium ions, so most of the calcium ions must come from outside the
cells. The result is a slower onset of contraction. Mitochondria are plentiful,
providing energy for the contractions of the heart. Typically,
cardiomyocytes have a single, central nucleus, but two or more nuclei may
be found in some cells.
Cardiac muscle cells branch freely. A junction between two adjoining cells
is marked by a critical structure called an intercalated disc, which helps
support the synchronized contraction of the muscle ([link]b). The
importance of strongly binding these cells together is necessitated by the
forces exerted by contraction.
Cardiac Muscle
Intercalated discs
Intercalated discs
Mitochondria Intercalated discs
Nucleus
__-<24
[ ——$——
Gap junction
Cardiac
muscle fiber
(a)
Desmosome
—
A band | band
(c)
(a) Cardiac muscle cells have intercalated discs that are
found at the junction of different cardiac muscle cells. (b) A
photomicrograph of cardiac muscle cells shows the nuclei
and intercalated discs. (Micrograph provided by the
Regents of the University of Michigan Medical School ©
2012)
Note:
Everyday Connection
Repair and Replacement
Damaged cardiac muscle cells have extremely limited abilities to repair
themselves or to replace dead cells via mitosis. Recent evidence indicates
that at least some stem cells remain within the heart that continue to divide
and at least potentially replace these dead cells. However, newly formed or
repaired cells are rarely as functional as the original cells, and cardiac
function is reduced. In the event of a heart attack or MI, dead cells are
often replaced by patches of scar tissue. Autopsies performed on
individuals who had successfully received heart transplants show some
proliferation of original cells. If researchers can unlock the mechanism that
generates new cells and restore full mitotic capabilities to heart muscle, the
prognosis for heart attack survivors will be greatly enhanced.
Conduction System of the Heart
If embryonic heart cells are separated into a Petri dish and kept alive, each
is capable of generating its own electrical impulse followed by contraction.
When two independently beating embryonic cardiac muscle cells are placed
together, the cell with the higher inherent rate sets the pace, and the impulse
spreads from the faster to the slower cell to trigger a contraction. As more
cells are joined together, the fastest cell continues to assume control of the
rate. A fully developed adult heart maintains the capability of generating its
own electrical impulse, triggered by the fastest cells, as part of the cardiac
conduction system. The components of the cardiac conduction system
include the sinoatrial (SA) node, the atrioventricular (AV) node, the
atrioventricular bundle, the atrioventricular bundle branches, and the
Purkinje fibers ([link]).
Conduction System of the Heart
AN ~>
Frontal plane
through heart
4
|
<< Arch of aorta
= -~
arma Bachman’s bundle
Sinoatrial
(SA) node
Anterior internodal
Atrioventricular
(AV) node
Middle internodal
Left atrium
Atrioventricular (AV)
bundle (bundle of His)
Posterior internodal
Left ventricle
Right and left bundle
branches
Right atrium
Right ventricle
Purkinje fibers
Anterior view of frontal section
Specialized conducting components of the heart include
the sinoatrial (SA) node, the atrioventricular (AV) node,
the atrioventricular bundle, the right and left bundle
branches, and the Purkinje fibers.
Sinoatrial (SA) Node
Normal cardiac rhythm is established by the sinoatrial (SA) node, a
specialized clump of myocardial conducting cells located in the superior
and posterior walls of the right atrium in close proximity to the opening of
the superior vena cava. The SA node is known as the pacemaker of the
heart. It initiates the sinus rhythm, or normal electrical pattern followed by
contraction of the heart.
This impulse spreads from its initiation in the SA node throughout the atria
to the atrioventricular (AV) node. The impulse takes approximately 50 ms
(milliseconds) to travel between these two nodes. The connective tissue of
the cardiac skeleton prevents the impulse from spreading into the
myocardial cells in the ventricles except at the atrioventricular node. [link]
illustrates the initiation of the impulse in the SA node that then spreads the
impulse throughout the atria to the atrioventricular node.
Cardiac Conduction
(1) The sinoatrial (SA) node and the
remainder of the conduction system are at
rest. (2) The SA node initiates the action
potential, which sweeps across the atria. (3)
After reaching the atrioventricular node,
there is a delay of approximately 100 ms
that allows the atria to complete pumping
blood before the impulse is transmitted to
the atrioventricular bundle. (4) Following
the delay, the impulse travels through the
atrioventricular bundle and bundle branches
to the Purkinje fibers. (5) The impulse
spreads to the contractile fibers of the
ventricle. (6) Ventricular contraction begins.
Atrioventricular (AV) Node
The atrioventricular (AV) node is a second clump of specialized
conductive cells, located in the lower portion of the right atrium within the
atrioventricular wall. There is a critical pause before the AV node initiates
an impulse and transmits it to the atrioventricular bundle (see [link], step 3).
Atrioventricular Bundle (Bundle of His), Bundle Branches, and
Purkinje Fibers
Arising from the AV node, the atrioventricular bundle, proceeds through
the septum before dividing into two atrioventricular bundle branches,
commonly called the left and right bundle branches. The left bundle branch
supplies the left ventricle, and the right bundle branch the right ventricle.
Both bundle branches descend and reach the apex of the heart where they
connect with the Purkinje fibers (see [link], step 4).
The Purkinje fibers are additional myocardial conductive fibers that spread
the impulse to the myocardial contractile cells in the ventricles. Since the
electrical stimulus begins at the apex, the contraction also begins at the apex
and travels toward the base of the heart, similar to squeezing a tube of
toothpaste from the bottom. This allows the blood to be pumped out of the
ventricles and into the aorta and pulmonary trunk.
Electrocardiogram
By careful placement of surface electrodes on the body, it is possible to
record the complex, compound electrical signal of the heart. This tracing of
the electrical signal is the electrocardiogram (ECG), also commonly
abbreviated EKG (K coming kardiology, from the German term for
cardiology). Careful analysis of the ECG reveals a detailed picture of both
normal and abnormal heart function, and is an indispensable clinical
diagnostic tool. The standard electrocardiograph (the instrument that
generates an ECG) uses 3, 5, or 12 leads. The greater the number of leads
an electrocardiograph uses, the more information the ECG provides. The
term “lead” may be used to refer to the cable from the electrode to the
electrical recorder, but it typically describes the voltage difference between
two of the electrodes. The 12-lead electrocardiograph uses 10 electrodes
placed in standard locations on the patient’s skin ([link]). In continuous
ambulatory electrocardiographs, the patient wears a small, portable, battery-
operated device known as a Holter monitor, that continuously monitors
heart electrical activity, typically for a period of 24 hours during the
patient’s normal routine.
Standard Placement of ECG Leads
In a 12-lead ECG, six electrodes are
placed on the chest, and four
electrodes are placed on the limbs.
A normal ECG tracing is presented in [link]. Each component, segment,
and interval is labeled and corresponds to important electrical events,
demonstrating the relationship between these events and contraction in the
heart.
There are five prominent points on the ECG: the P wave, the QRS complex,
and the T wave. The small P wave represents the depolarization of the atria.
The atria begin contracting approximately 25 ms after the start of the P
wave. The large QRS complex represents the depolarization of the
ventricles, which requires a much stronger electrical signal because of the
larger size of the ventricular cardiac muscle. The ventricles begin to
contract as the QRS reaches the peak of the R wave. Lastly, the T wave
represents the repolarization of the ventricles. The repolarization of the atria
occurs during the QRS complex, which masks it on an ECG.
Note:
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Visit this site for a more detailed analysis of ECGs.
Electrocardiogram
P-R S-T
segment segment
Millivolts
QRS complex
PR interval QT interval
A normal tracing shows the P wave, QRS
complex, and T wave. Additional segments and
intervals are also shown.
ECG Tracing Correlated to the Cardiac Cycle
=a
This diagram correlates an ECG tracing with the
electrical and mechanical events of a heart contraction.
Each segment of an ECG tracing corresponds to one
event in the cardiac cycle.
Note:
Everyday Connection
While interpretation of an ECG is possible and extremely valuable after
some training, a full understanding of the complexities and intricacies
generally requires several years of experience. In general, the size of the
electrical variations, the duration of the events, and detailed analysis
provide the most comprehensive picture of cardiac function. For example,
an amplified P wave may indicate enlargement of the atria and an enlarged
Q wave may indicate a MI (Myocardial Infarction). T waves often appear
flatter when insufficient oxygen is being delivered to the myocardium.
As useful as analyzing these electrical recordings may be, there are
limitations. For example, not all areas suffering a MI may be obvious on
the ECG. Additionally, it will not reveal the effectiveness of the pumping,
which requires further testing, such as an ultrasound test called an
echocardiogram or nuclear medicine imaging. Common abnormalities that
may be detected by the ECGs are shown in [link].
Common ECG Abnormalities
Second-degree (partial) block
Atrial fibrillation
Ventricular tachycardia
Ventricular fibrillation
Third-degree block
Note how half of the P waves
are not followed by the QRS
complex and T waves while the
other half are.
Question: What would you
expect to happen to heart rate
(pulse)?
Note the abnormal electrical
pattern prior to the QRS
complexes. Also note how the
frequency between the QRS
complexes has increased.
Question: What would you
expect to happen to heart rate
(pulse)?
Note the unusual shape of the
QRS complex, focusing on the
“S” component.
Question: What would you
expect to happen to heart rate
(pulse)?
Note the total lack of normal
electrical activity.
Question: What would you
expect to happen to heart
rate (pulse)?
Note that in a third-degree block
some of the impulses initiated by
the SA node do not reach the
AV node while others do. Also note
that the P waves are not followed
by the QRS complex.
Question: What would you expect
to happen to heart rate (pulse)?
(a) In a second-degree or partial block, one-half of the P
waves are not followed by the QRS complex and T waves
while the other half are. (b) In atrial fibrillation, the
electrical pattern is abnormal prior to the QRS complex,
and the frequency between the QRS complexes has
increased. (c) In ventricular tachycardia, the shape of the
QRS complex is abnormal. (d) In ventricular fibrillation,
there is no normal electrical activity. (e) In a third-degree
block, there is no correlation between atrial activity (the P
wave) and ventricular activity (the QRS complex).
Note:
Everyday Connection
External Automated Defibrillators
In the event that the electrical activity of the heart is severely disrupted,
cessation of electrical activity or fibrillation may occur. In fibrillation, the
heart beats in a wild, uncontrolled manner, which prevents it from being
able to pump effectively. Atrial fibrillation (see [link]b) is a serious
condition, but as long as the ventricles continue to pump blood, the
patient’s life may not be in immediate danger. Ventricular fibrillation (see
[link ]d) is a medical emergency that requires life support, because the
ventricles are not effectively pumping blood. In a hospital setting, it is
often described as “code blue.” If untreated for as little as a few minutes,
ventricular fibrillation may lead to brain death. The most common
treatment is defibrillation, which uses special paddles to apply a charge to
the heart from an external electrical source in an attempt to establish a
normal sinus rhythm ([link]). A defibrillator effectively stops the heart so
that the SA node can trigger a normal conduction cycle. Because of their
effectiveness in reestablishing a normal sinus rhythm, external automated
defibrillators (EADs) are being placed in areas frequented by large
numbers of people, such as schools, restaurants, and airports. These
devices contain simple and direct verbal instructions that can be followed
by nonmedical personnel in an attempt to save a life.
Defibrillators
(a) An external automatic defibrillator can be used by
nonmedical personnel to reestablish a normal sinus rhythm
in a person with fibrillation. (b) Defibrillator paddles are
more commonly used in hospital settings. (credit b:
“widerider107”/flickr.com)
When arrhythmias become a chronic problem, the heart maintains a
junctional rhythm, which originates in the AV node. In order to speed up the
heart rate and restore full sinus rhythm, a cardiologist can implant an
artificial pacemaker, which delivers electrical impulses to the heart
muscle to ensure that the heart continues to contract and pump blood
effectively. These artificial pacemakers are programmable by the
cardiologists and can either provide stimulation temporarily upon demand
or on a continuous basis. Some devices also contain built-in defibrillators.
Chapter Review
The heart is regulated by both neural and endocrine (i.e. hormonal) control,
yet it is capable of initiating its own action potential followed by muscular
contraction. The conductive cells within the heart establish the heart rate
and transmit it through the myocardium. The contractile cells contract and
propel the blood. The normal path of transmission for the conductive cells
is the sinoatrial (SA) node, atrioventricular (AV) node, atrioventricular (AV)
bundle, bundle branches, and Purkinje fibers. Recognizable points on the
ECG include the P wave that corresponds to atrial depolarization (i.e.
contraction), the QRS complex that corresponds to ventricular
depolarization, and the T wave that corresponds to ventricular
repolarization (relaxation).
Review Questions
Exercise:
Problem: Which of the following is unique to cardiac muscle cells?
a. Only cardiac muscle contains a sarcoplasmic reticulum.
b. Only cardiac muscle has gap junctions.
c. Only cardiac muscle is capable of autorhythmicity
d. Only cardiac muscle has a high concentration of mitochondria.
Solution:
C
Exercise:
Problem:
Which portion of the ECG corresponds to repolarization (i.e.
relaxation) of the atria?
a. P wave
b. QRS complex
c. T wave
d. none of the above: atrial repolarization is masked by ventricular
depolarization
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
How does the delay of the impulse at the atrioventricular node
contribute to cardiac function?
Solution:
It ensures sufficient time for the atrial muscle to contract and pump
blood into the ventricles prior to the impulse being conducted into the
lower chambers.
Glossary
artificial pacemaker
medical device that transmits electrical signals to the heart to ensure
that it contracts and pumps blood to the body
atrioventricular bundle
(also, bundle of His) group of specialized myocardial conductile cells
that transmit the impulse from the AV node through the
interventricular septum; form the left and right atrioventricular bundle
branches
atrioventricular bundle branches
(also, left or right bundle branches) specialized myocardial conductile
cells that arise from the bifurcation of the atrioventricular bundle and
pass through the interventricular septum; lead to the Purkinje fibers
and also to the right papillary muscle via the moderator band
atrioventricular (AV) node
clump of myocardial cells located in the inferior portion of the right
atrium within the atrioventricular septum; receives the impulse from
the SA node, pauses, and then transmits it into specialized conducting
cells within the interventricular septum
autorhythmicity
ability of cardiac muscle to initiate its own electrical impulse that
triggers the mechanical contraction that pumps blood at a fixed pace
without nervous or endocrine control
Bachmann’s bundle
(also, interatrial band) group of specialized conducting cells that
transmit the impulse directly from the SA node in the right atrium to
the left atrium
bundle of His
(also, atrioventricular bundle) group of specialized myocardial
conductile cells that transmit the impulse from the AV node through
the interventricular septum; form the left and right atrioventricular
bundle branches
electrocardiogram (ECG)
surface recording of the electrical activity of the heart that can be used
for diagnosis of irregular heart function; also abbreviated as EKG
heart block
interruption in the normal conduction pathway
interatrial band
(also, Bachmann’s bundle) group of specialized conducting cells that
transmit the impulse directly from the SA node in the right atrium to
the left atrium
intercalated disc
physical junction between adjacent cardiac muscle cells; consisting of
desmosomes, specialized linking proteoglycans, and gap junctions that
allow passage of ions between the two cells
internodal pathways
specialized conductile cells within the atria that transmit the impulse
from the SA node throughout the myocardial cells of the atrium and to
the AV node
myocardial conducting cells
specialized cells that transmit electrical impulses throughout the heart
and trigger contraction by the myocardial contractile cells
myocardial contractile cells
bulk of the cardiac muscle cells in the atria and ventricles that conduct
impulses and contract to propel blood
P wave
component of the electrocardiogram that represents the depolarization
of the atria
pacemaker
cluster of specialized myocardial cells known as the SA node that
initiates the sinus rhythm
prepotential depolarization
(also, spontaneous depolarization) mechanism that accounts for the
autorhythmic property of cardiac muscle; the membrane potential
increases as sodium ions diffuse through the always-open sodium ion
channels and causes the electrical potential to rise
Purkinje fibers
specialized myocardial conduction fibers that arise from the bundle
branches and spread the impulse to the myocardial contraction fibers
of the ventricles
QRS complex
component of the electrocardiogram that represents the depolarization
of the ventricles and includes, as a component, the repolarization of the
atria
sinoatrial (SA) node
known as the pacemaker, a specialized clump of myocardial
conducting cells located in the superior portion of the right atrium that
has the highest inherent rate of depolarization that then spreads
throughout the heart
sinus rhythm
normal contractile pattern of the heart
spontaneous depolarization
(also, prepotential depolarization) the mechanism that accounts for the
autorhythmic property of cardiac muscle; the membrane potential
increases as sodium ions diffuse through the always-open sodium ion
channels and causes the electrical potential to rise
T wave
component of the electrocardiogram that represents the repolarization
of the ventricles
Cardiac Cycle
By the end of this section, you will be able to:
e Describe the relationship between blood pressure and blood flow
e Summarize the events of the cardiac cycle
e¢ Compare atrial and ventricular systole and diastole
e Relate heart sounds detected by auscultation to action of heart’s valves
The period of time that begins with contraction of the atria and ends with
ventricular relaxation is known as the cardiac cycle ([link]). The period of
contraction that the heart undergoes while it pumps blood into circulation is
called systole. The period of relaxation that occurs as the chambers fill with
blood is called diastole. Both the atria and ventricles undergo systole and
diastole, and it is essential that these components be carefully regulated and
coordinated to ensure blood is pumped efficiently to the body.
Overview of the Cardiac Cycle
|
The cardiac cycle begins with atrial systole and
progresses to ventricular systole, atrial
diastole, and ventricular diastole, when the
cycle begins again. Correlations to the ECG
are highlighted.
Pressures and Flow
Fluids, whether gases or liquids, are materials that flow according to
pressure gradients—that is, they move from regions that are higher in
pressure to regions that are lower in pressure. Accordingly, when the heart
chambers are relaxed (diastole), blood will flow into the atria from the
veins, which are higher in pressure. As blood flows into the atria, the
pressure will rise, so the blood will initially move passively from the atria
into the ventricles. When the action potential triggers the muscles in the
atria to contract (atrial systole), the pressure within the atria rises further,
pumping blood into the ventricles. During ventricular systole, pressure rises
in the ventricles, pumping blood into the pulmonary trunk from the right
ventricle and into the aorta from the left ventricle. Again, as you consider
this flow and relate it to the conduction pathway, the elegance of the system
should become apparent.
Phases of the Cardiac Cycle
At the beginning of the cardiac cycle, both the atria and ventricles are
relaxed (diastole). Blood is flowing into the right atrium from the superior
and inferior venae cavae and the coronary sinus. Blood flows into the left
atrium from the four pulmonary veins. The two atrioventricular valves, the
tricuspid and mitral valves, are both open, so blood flows unimpeded from
the atria and into the ventricles. Approximately 70—80 percent of ventricular
filling occurs by this method. The two semilunar valves, the pulmonary and
aortic valves, are closed, preventing backflow of blood into the right and
left ventricles from the pulmonary trunk on the right and the aorta on the
left.
Atrial Systole and Diastole
Contraction of the atria follows depolarization, represented by the P wave
of the ECG. As the atrial muscles contract from the superior portion of the
atria toward the atrioventricular septum, pressure rises within the atria and
blood is pumped into the ventricles through the open atrioventricular
(tricuspid, and mitral or bicuspid) valves. At the start of atrial systole, the
ventricles are normally filled with approximately 70-80 percent of their
capacity due to inflow during diastole. Atrial contraction, also referred to as
the “atrial kick,” contributes the remaining 20—30 percent of filling (see
[link]). Atrial systole ends prior to ventricular systole, as the atrial muscle
returns to diastole.
Ventricular Systole
Ventricular systole (see [link]) follows the depolarization of the ventricles
and is represented by the QRS complex in the ECG. It may be conveniently
divided into two phases, lasting a total of 270 ms. At the end of atrial
systole and just prior to atrial contraction, the ventricles contain
approximately 130 mL blood in a resting adult in a standing position. This
volume is known as the end diastolic volume (EDV) or preload.
Initially, as the muscles in the ventricle contract, the pressure of the blood
within the chamber rises, but it is not yet high enough to open the semilunar
(pulmonary and aortic) valves and be ejected from the heart. However,
blood pressure quickly rises above that of the atria that are now relaxed and
in diastole. This increase in pressure causes blood to flow back toward the
atria, closing the tricuspid and mitral valves.
In the second phase of ventricular systole, the contraction of the ventricular
muscle has raised the pressure within the ventricle to the point that it is
greater than the pressures in the pulmonary trunk and the aorta. Blood is
pumped from the heart, pushing open the pulmonary and aortic semilunar
valves. Pressure generated by the left ventricle will be appreciably greater
than the pressure generated by the right ventricle, since the existing
pressure in the aorta will be so much higher. Nevertheless, both ventricles
pump the same amount of blood.
Ventricular Diastole
Ventricular relaxation, or diastole, follows repolarization of the ventricles
and is represented by the T wave of the ECG.
During the early phase of ventricular diastole, as the ventricular muscle
relaxes, pressure on the remaining blood within the ventricle begins to fall.
When pressure within the ventricles drops below pressure in both the
pulmonary trunk and aorta, blood flows back toward the heart. The
semilunar valves close to prevent backflow into the heart.
In the second phase of ventricular diastole, as the ventricular muscle
relaxes, pressure on the blood within the ventricles drops even further.
Eventually, it drops below the pressure in the atria. When this occurs, blood
flows from the atria into the ventricles, pushing open the tricuspid and
mitral valves. As pressure drops within the ventricles, blood flows from the
major veins into the relaxed atria and from there into the ventricles. Both
chambers are in diastole, the atrioventricular valves are open, and the
semilunar valves remain closed (see [link]). The cardiac cycle is complete.
[link] illustrates the relationship between the cardiac cycle and the ECG.
Relationship between the Cardiac Cycle and ECG
R
po Nenireslenaeriies =
One cardiac cycle
Initially, both the atria and ventricles are relaxed
(diastole). The P wave represents depolarization of the
atria and is followed by atrial contraction (systole).
Atrial systole extends until the QRS complex, at
which point, the atria relax. The QRS complex
represents depolarization of the ventricles and is
followed by ventricular contraction. The T wave
represents the repolarization of the ventricles and
marks the beginning of ventricular relaxation.
Heart Sounds
One of the simplest, yet effective, diagnostic techniques applied to assess
the state of a patient’s heart is auscultation using a stethoscope.
In a normal, healthy heart, there are only two audible heart sounds: Lub
and Dup (or Dub). “Lub,” or first heart sound is the sound created by the
closing of the atrioventricular valves during ventricular contraction . The
second heart sound,"Dup" (or "Dub") is the sound of the closing of the
semilunar valves during ventricular diastole.
The term murmur is used to describe an unusual sound coming from the
heart that is caused by the turbulent flow of blood. Murmurs are graded on a
scale of 1 to 6, with 1 being the most common, the most difficult sound to
detect, and the least serious. The most severe is a 6. Specialized electronic
stethoscopes are used to record both normal and abnormal sounds.
When using a stethoscope to listen to the heart sounds, called asculation, it
is common practice for the clinician to ask the patient to breathe deeply.
This procedure not only allows for listening to airflow, but it may also
amplify heart murmurs. Inhalation increases blood flow into the right side
of the heart and may increase the amplitude of right-sided heart murmurs.
Expiration partially restricts blood flow into the left side of the heart and
may amplify left-sided heart murmurs. [link] indicates proper placement of
the bell of the stethoscope to facilitate hearing the sounds.
Stethoscope Placement for Auscultation
Aortic valve Pulmonary valve
Tricuspid valve Mitral valve
Proper placement of the bell of the stethoscope
facilitates auscultation. At each of the four locations
on the chest, a different valve can be heard.
Chapter Review
The cardiac cycle comprises a complete relaxation and contraction of both
the atria and ventricles, and lasts approximately 0.8 seconds. Beginning
with all chambers in diastole, blood flows passively from the veins into the
atria and past the atrioventricular valves into the ventricles. The atria begin
to contract (atrial systole), following depolarization of the atria, and pump
blood into the ventricles. The ventricles begin to contract (ventricular
systole), raising pressure within the ventricles. When ventricular pressure
rises above the pressure in the atria, blood flows toward the atria, producing
the first heart sound, lub. As pressure in the ventricles rises above two
major arteries, blood pushes open the two semilunar valves and moves into
the pulmonary trunk and aorta in the ventricular ejection phase. Following
ventricular repolarization, the ventricles begin to relax (ventricular
diastole), and pressure within the ventricles drops. As ventricular pressure
drops, there is a tendency for blood to flow back into the atria from the
major arteries, closing the two semilunar valves. The second heart sound,
dub (or dup) , occurs when the semilunar valves close. When the pressure
falls below that of the atria, blood moves from the atria into the ventricles,
opening the atrioventricular valves and marking one complete heart cycle.
The valves prevent backflow of blood. Failure of the valves to operate
properly produces turbulent blood flow within the heart; the resulting heart
murmur can often be heard with a stethoscope.
Review Questions
Exercise:
Problem: Most blood enters the ventricle during
a. atrial systole
b. atrial diastole
c. ventricular systole
d. isovolumic contraction
Solution:
B
Exercise:
Problem:
The first heart sound represents which portion of the cardiac cycle?
a. atrial systole
b. ventricular systole
c. closing of the atrioventricular valves
d. closing of the semilunar valves
Solution:
C
Exercise:
Problem: Ventricular relaxation immediately follows
a. atrial depolarization
b. ventricular repolarization
c. ventricular depolarization
d. atrial repolarization
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Describe one cardiac cycle, beginning with both atria and ventricles
relaxed.
Solution:
The cardiac cycle comprises a complete relaxation and contraction of
both the atria and ventricles, and lasts approximately 0.8 seconds.
Beginning with all chambers in diastole, blood flows passively from
the veins into the atria and past the atrioventricular valves into the
ventricles. The atria begin to contract following depolarization of the
atria and pump blood into the ventricles. The ventricles begin to
contract, raising pressure within the ventricles. When ventricular
pressure rises above the pressure in the two major arteries, blood
pushes open the two semilunar valves and moves into the pulmonary
trunk and aorta in the ventricular ejection phase. Following ventricular
repolarization, the ventricles begin to relax, and pressure within the
ventricles drops. When the pressure falls below that of the atria, blood
moves from the atria into the ventricles, opening the atrioventricular
valves and marking one complete heart cycle.
Glossary
cardiac cycle
period of time between the onset of atrial contraction (atrial systole)
and ventricular relaxation (ventricular diastole)
diastole
period of time when the heart muscle is relaxed and the chambers fill
with blood
end diastolic volume (EDV)
(also, preload) the amount of blood in the ventricles at the end of atrial
systole just prior to ventricular contraction
end systolic volume (ESV)
amount of blood remaining in each ventricle following systole
heart sounds
sounds heard via auscultation with a stethoscope of the closing of the
atrioventricular valves (“lub”) and semilunar valves (“dub”’)
isovolumic contraction
(also, isovolumetric contraction) initial phase of ventricular
contraction in which tension and pressure in the ventricle increase, but
no blood is pumped or ejected from the heart
isovolumic ventricular relaxation phase
initial phase of the ventricular diastole when pressure in the ventricles
drops below pressure in the two major arteries, the pulmonary trunk,
and the aorta, and blood attempts to flow back into the ventricles,
producing the dicrotic notch of the ECG and closing the two semilunar
valves
murmur
unusual heart sound detected by auscultation; typically related to septal
or valve defects
preload
(also, end diastolic volume) amount of blood in the ventricles at the
end of atrial systole just prior to ventricular contraction
systole
period of time when the heart muscle is contracting
ventricular ejection phase
second phase of ventricular systole during which blood is pumped
from the ventricle
Introduction to the Cardiovascular System - Blood Vessels and Circulation
class="introduction"
Blood Vessels
While most
blood
vessels are
located
deep from
the surface
and are not
visible, the
superficial
veins of the
upper limb
provide an
indication
of the
extent,
prominence
, and
importance
of these
structures to
the body.
(credit:
Colin
Davis)
Note:
Chapter Objectives
After studying this chapter, you will be able to:
e Compare and contrast the anatomical structure of arteries, arterioles,
capillaries, venules, and veins
e Accurately describe the forces that account for capillary exchange
e Describe the interaction of the cardiovascular system with other body
systems
e Identify and describe the hepatic portal system
In this chapter, you will learn about the vascular part of the cardiovascular
system, that is, the vessels that transport blood throughout the body and
provide the physical site where gases, nutrients, and other substances are
exchanged with body cells. When vessel functioning is reduced, blood-
borne substances do not circulate effectively throughout the body. As a
result, tissue injury occurs, metabolism is impaired, and the functions of
every bodily system are threatened.
Structure and Function of Blood Vessels
By the end of this section, you will be able to:
e Distinguish between arteries, arterioles, capillaries, venules, and veins
with respect to basic structure and function
Blood is carried through the body via blood vessels. An artery is a blood
vessel that carries blood away from the heart, where it branches into ever-
smaller vessels. Eventually, the smallest arteries, vessels called arterioles,
further branch into tiny capillaries, where nutrients and wastes are
exchanged, and then combine with other vessels that exit capillaries to form
venules, small blood vessels that carry blood to a vein, a larger blood vessel
that returns blood to the heart.
Arteries and veins transport blood in two distinct circuits: the systemic
circuit and the pulmonary circuit ([link]). Systemic arteries provide blood
rich in oxygen to the body’s tissues. The blood returned to the heart through
systemic veins has less oxygen, since much of the oxygen carried by the
arteries has been delivered to the cells. In contrast, in the pulmonary circuit,
arteries carry blood low in oxygen exclusively to the lungs for gas
exchange. Pulmonary veins then return freshly oxygenated blood from the
lungs to the heart to be pumped back out into systemic circulation.
Although arteries and veins differ structurally and functionally, they share
certain features.
Cardiovascular Circulation
Lungs
2c
GO :
5 & Pulmonary Pulmonary vein
=)
§3 artery
as
Vena cava Aorta
Upper body
Liver
Hepatic vein Hepatic artery
Hepatic portal vein
Systemic
circulation
Stomach,
intestines Hi Vessels transporting
oxygenated blood
Renal artery BH Vessels transporting
Renal vein
deoxygenated blood
Midneye Bi Vessels involved in
gas excange
Lower body
The pulmonary circuit moves blood from the right side of
the heart to the lungs and back to the heart. The systemic
circuit moves blood from the left side of the heart to the
head and body and returns it to the right side of the heart to
repeat the cycle. The arrows indicate the direction of blood
flow, and the colors show the relative levels of oxygen
concentration.
Shared Structures
Different types of blood vessels vary slightly in their structures, but they
share the same general features. Arteries and arterioles have thicker walls
than veins and venules because they are closer to the heart and receive
blood that is surging at a far greater pressure ({link]). Each type of vessel
has a lumen—a hollow passageway through which blood flows. Arteries
have smaller lumens than veins, a characteristic that helps to maintain the
pressure of blood moving through the system. Together, their thicker walls
and smaller diameters give arterial lumens a more rounded appearance in
cross section than the lumens of veins.
Structure of Blood Vessels
Artery Vein
=~ 7} Tunica externa
Baa in Tunica externa
s im Tunica media
Tunica intima
Vasa vasorum
Tunica media
7 I Tunica intima
vi uA
Smooth muscle
Internal elastic
membrane Smooth muscle
Vasa vasorum
External elastic
membrane
Nervi vasorum
Endothelium
Elastic fiber
= Endothelium
\
(a) Arteries and (b) veins share the same
general features, but the walls of arteries
are much thicker because of the higher
pressure of the blood that flows through
them. (c) A micrograph shows the
relative differences in thickness. LM x
160. (Micrograph provided by the
Regents of the University of Michigan
Medical School © 2012)
By the time blood has passed through capillaries and entered venules, the
pressure initially exerted upon it by heart contractions has diminished. In
other words, in comparison to arteries, venules and veins withstand a much
lower pressure from the blood that flows through them. Their walls are
considerably thinner and their lumens are correspondingly larger in
diameter, allowing more blood to flow with less vessel resistance. In
addition, many veins of the body, particularly those of the limbs, contain
valves that assist the unidirectional flow of blood toward the heart. This is
critical because blood flow becomes sluggish in the extremities, as a result
of the lower pressure and the effects of gravity.
Capillaries
A capillary is a microscopic channel that supplies blood to the tissues
themselves. Exchange of gases and other substances occurs in the
capillaries between the blood and the surrounding cells and their tissue fluid
(interstitial fluid). The diameter of a capillary lumen ranges from 5—10
micrometers; the smallest are just barely wide enough for an erythrocyte to
squeeze through.
Chapter Review
Blood pumped by the heart flows through a series of vessels known as
arteries, arterioles, capillaries, venules, and veins before returning to the
heart. Arteries transport blood away from the heart and branch into smaller
vessels, forming arterioles. Arterioles distribute blood to capillary beds, the
sites of exchange with the body tissues. Capillaries lead back to small
vessels known as venules that flow into the larger veins and eventually back
to the heart.
The arterial system is a relatively high-pressure system, so arteries have
thick walls with more elastic fibers that appear round in cross section. The
venous system is a lower-pressure system, containing veins that have larger
lumens and thinner walls. They often appear flattened.
Review Questions
Exercise:
Problem:
Closer to the heart, arteries would be expected to have a higher
percentage of to help deal with the increased pressure.
a. endothelium
b. smooth muscle fibers
c. elastic fibers
d. collagenous fibers
Solution:
CG
Exercise:
Problem: Which of the following best describes veins?
a. thick walled, small lumens, low pressure, lack valves
b. thin walled, large lumens, low pressure, have valves
c. thin walled, small lumens, high pressure, have valves
d. thick walled, large lumens, high pressure, lack valves
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Cocaine use causes vasoconstriction. Is this likely to increase or
decrease blood pressure, and why?
Solution:
Vasoconstriction causes the lumens of blood vessels to narrow. This
increases the pressure of the blood flowing within the vessel.
Glossary
arteriole
(also, resistance vessel) very small artery that leads to a capillary
arteriovenous anastomosis
short vessel connecting an arteriole directly to a venule and bypassing
the capillary beds
artery
blood vessel that conducts blood away from the heart; may be a
conducting or distributing vessel
Capacitance
ability of a vein to distend and store blood
Capacitance vessels
veins
capillary
smallest of blood vessels where physical exchange occurs between the
blood and tissue cells surrounded by interstitial fluid
capillary bed
network of 10-100 capillaries connecting arterioles to venules
continuous capillary
most common type of capillary, found in virtually all tissues except
epithelia and cartilage; contains very small gaps in the endothelial
lining that permit exchange
elastic artery
(also, conducting artery) artery with abundant elastic fibers located
closer to the heart, which maintains the pressure gradient and conducts
blood to smaller branches
external elastic membrane
membrane composed of elastic fibers that separates the tunica media
from the tunica externa; seen in larger arteries
fenestrated capillary
type of capillary with pores or fenestrations in the endothelium that
allow for rapid passage of certain small materials
internal elastic membrane
membrane composed of elastic fibers that separates the tunica intima
from the tunica media; seen in larger arteries
lumen
interior of a tubular structure such as a blood vessel or a portion of the
alimentary canal through which blood, chyme, or other substances
travel
metarteriole
short vessel arising from a terminal arteriole that branches to supply a
capillary bed
microcirculation
blood flow through the capillaries
muscular artery
(also, distributing artery) artery with abundant smooth muscle in the
tunica media that branches to distribute blood to the arteriole network
nervi vasorum
small nerve fibers found in arteries and veins that trigger contraction of
the smooth muscle in their walls
perfusion
distribution of blood into the capillaries so the tissues can be supplied
precapillary sphincters
circular rings of smooth muscle that surround the entrance to a
capillary and regulate blood flow into that capillary
sinusoid capillary
rarest type of capillary, which has extremely large intercellular gaps in
the basement membrane in addition to clefts and fenestrations; found
in areas such as the bone marrow and liver where passage of large
molecules occurs
thoroughfare channel
continuation of the metarteriole that enables blood to bypass a
capillary bed and flow directly into a venule, creating a vascular shunt
tunica externa
(also, tunica adventitia) outermost layer or tunic of a vessel (except
capillaries)
tunica intima
(also, tunica interna) innermost lining or tunic of a vessel
tunica media
middle layer or tunic of a vessel (except capillaries)
vasa vasorum
small blood vessels located within the walls or tunics of larger vessels
that supply nourishment to and remove wastes from the cells of the
vessels
vascular shunt
continuation of the metarteriole and thoroughfare channel that allows
blood to bypass the capillary beds to flow directly from the arterial to
the venous circulation
vasoconstriction
constriction of the smooth muscle of a blood vessel, resulting in a
decreased vascular diameter
vasodilation
relaxation of the smooth muscle in the wall of a blood vessel, resulting
in an increased vascular diameter
vasomotion
irregular, pulsating flow of blood through capillaries and related
structures
vein
blood vessel that conducts blood toward the heart
venous reserve
volume of blood contained within systemic veins in the integument,
bone marrow, and liver that can be returned to the heart for circulation,
if needed
venule
small vessel leading from the capillaries to veins
Introduction to the Respiratory System
class="introduction"
Mountain Climbers
The thin air at high
elevations can strain the
human respiratory system.
(credit:
“bortescristian”/flickr.com
)
Note:
Chapter Objectives
After studying this chapter, you will be able to:
e List the structures of the respiratory system
e List the major functions of the respiratory system
e Outline the forces that allow for air movement into and out of the
lungs
e Outline the process of gas exchange
e Summarize the process of oxygen and carbon dioxide transport within
the respiratory system
e Discuss how the respiratory system responds to exercise
Hold your breath. Really! See how long you can hold your breath as you
continue reading...How long can you do it? Chances are you are feeling
uncomfortable already. A typical human cannot survive without breathing
for more than 3 minutes, and even if you wanted to hold your breath longer,
your autonomic nervous system would take control. This is because every
cell in the body needs to run the oxidative stages of cellular respiration, the
process by which energy is produced in the form of adenosine triphosphate
(ATP). For oxidative phosphorylation to occur, oxygen is used as a reactant
and carbon dioxide is released as a waste product. You may be surprised to
learn that although oxygen is a critical need for cells, it is actually the
accumulation of carbon dioxide that primarily drives your need to breathe.
Carbon dioxide is exhaled and oxygen is inhaled through the respiratory
system, which includes muscles to move air into and out of the lungs,
passageways through which air moves, and microscopic gas exchange
surfaces covered by capillaries. The circulatory system transports gases
from the lungs to tissues throughout the body and vice versa. A variety of
diseases can affect the respiratory system, such as asthma, emphysema,
chronic obstruction pulmonary disorder (COPD), and lung cancer. All of
these conditions affect the gas exchange process and result in labored
breathing and other difficulties.
Organs and Structures of the Respiratory System
By the end of this section, you will be able to:
e List the structures that make up the respiratory system
e Describe how the respiratory system processes oxygen and CO»
e Compare and contrast the functions of upper respiratory tract with the
lower respiratory tract
The major organs of the respiratory system function primarily to provide
oxygen to body tissues for cellular respiration, remove the waste product
carbon dioxide, and help to maintain acid-base balance. Portions of the
respiratory system are also used for non-vital functions, such as sensing
odors, speech production, and coughing ([link]).
Major Respiratory Structures
Nasal cavity
Nostril
Oral cavit
: Pharynx
Larynx
Trachea
Left main
bronchus
Right main
bronchus
Right lun
? 3 Left lung
Diaphragm
The major respiratory structures span the nasal
cavity to the diaphragm.
The Nose and its Adjacent Structures
The major entrance and exit for the respiratory system is through the nose,
via the nostrils. The inhaled air enters into the nasal cavity, which is
separated into left and right sections by the nasal septum. The wall of the
nasal cavity has three bony projections, called the superior, middle, and
inferior nasal conchae. Conchae serve to increase the surface area of the
nasal cavity and to disrupt the flow of air as it enters the nose, causing air to
bounce along the epithelium, where it is filtered, warmed, and humidified.
Air exits the nasal cavities and moves into the pharynx.
Several bones that help form the walls of the nasal cavity have air-
containing spaces called the sinuses, which serve to warm and humidify
incoming air. Sinuses are lined with a mucosa. Each sinus is named for its
associated bone: frontal sinus, maxillary sinus, sphenoidal sinus, and
ethmoidal sinus. The sinuses produce mucus and lighten the weight of the
skull.
Portions of the nasal cavities are lined with mucous membranes, containing
sebaceous glands and hair follicles that serve to prevent the passage of large
debris, such as dirt, through the nasal cavity.
The conchae and sinuses are lined by respiratory epithelium composed of
pseudostratified ciliated columnar epithelium ([link]). The epithelium
contains goblet cells, one of the specialized, columnar epithelial cells that
produce mucus to trap debris. The cilia of the respiratory epithelium help
remove the mucus and debris from the nasal cavity with a constant beating
motion, sweeping materials towards the throat to be swallowed.
Interestingly, cold air slows the movement of the cilia, resulting in
accumulation of mucus that may in turn lead to a runny nose during cold
weather. This moist epithelium functions to warm and humidify incoming
air. Capillaries located just beneath the nasal epithelium warm the incoming
air.
Pseudostratified Ciliated Columnar Epithelium
Lumen of
Goblet cell trachea Cilia
Pseudostratified
columnar epithelia
Seromucous gland
in submucosa
Respiratory epithelium is pseudostratified ciliated
columnar epithelium. Seromucous glands provide
lubricating mucus. LM x 680. (Micrograph provided by
the Regents of University of Michigan Medical School ©
2012)
Pharynx
The pharynx is a tube formed by skeletal muscle and lined by mucous
membrane that is continuous with that of the nasal cavities. The pharynx is
divided into three major regions: the nasopharynx, the oropharynx, and the
laryngopharynx ([Link]).
Divisions of the Pharynx
Nasal cavity
Hard palate Soft palate
Tongue
Epiglottis
Larynx (voice box)
Esophagus
Trachea
The pharynx is divided into three regions: the
nasopharynx, the oropharynx, and the
laryngopharynx.
The nasopharynx is flanked by the conchae of the nasal cavity, and it
serves only as an airway. At the top of the nasopharynx are the pharyngeal
tonsils. A pharyngeal tonsil, also called an adenoid, is a collection of tissue
similar to a lymph node that lies at the top portion of the nasopharynx. The
function of the pharyngeal tonsil is not well understood, but it contains a
rich supply of lymphocytes (a type of WBC) and is covered with ciliated
epithelium that traps and destroys invading pathogens that enter during
inhalation. The pharyngeal tonsils are large in children, but interestingly,
tend to regress with age and may even disappear. The uvula is a small
bulbous, teardrop-shaped structure located at the apex of the soft palate.
Both the uvula and soft palate move like a pendulum during swallowing,
swinging upward to close off the nasopharynx to prevent ingested materials
from entering the nasal cavity. In addition, auditory (Eustachian) tubes that
connect to each middle ear cavity open into the nasopharynx. This
connection is why colds often lead to ear infections.
The oropharynx is a passageway for both air and food. It contains two
distinct sets of tonsils, the palatine and lingual tonsils. Similar to the
pharyngeal tonsil, the palatine and lingual tonsils are composed of
lymphoid tissue, and trap and destroy pathogens entering the body through
the oral or nasal cavities.
The laryngopharynx continues the route for ingested material and air until
its inferior end, where the digestive and respiratory systems split. To the
front,the laryngopharynx opens into the larynx, whereas to the back, it
enters the esophagus.
Larynx
The larynx is a structure below the laryngopharynx that connects the
pharynx to the trachea and helps regulate the volume of air that enters and
leaves the lungs ({link]). The structure of the larynx is formed by several
pieces of cartilage.
Larynx
Epiglottis
Body of hyoid bone
Thyrohyoid membrane
Thyroid cartilage
Laryngeal prominence
Cricothyroid ligament
Y q Cricoid cartilage
Cricotracheal ligament
Tracheal cartilages
Epiglottis
Thyrohyoid membrane
Body of hyoid bone
Fatty pad
Thyrohyoid membrane
Vestibular fold
Vocal fold
Thyroid cartilage
Cricothyroid ligament
Cuneiform cartilage
Corniculate cartilage
Arytenoid cartilage
Cricoid cartilage
Cricotracheal ligament
Tracheal cartilages
Right lateral view
The larynx extends from the laryngopharynx and
the hyoid bone to the trachea.
The epiglottis, attached to the thyroid cartilage, is a very flexible piece of
elastic cartilage that covers the opening of the trachea (see [link]). When in
the “closed” position, the unattached end of the epiglottis rests on the
glottis. The glottis is composed of the vestibular folds, the true vocal cords,
and the space between these folds ([link]). The inner edges of the true vocal
cords are free, allowing oscillation to produce sound. The size of the
membranous folds of the true vocal cords differs between individuals,
producing voices with different pitch ranges. Folds in males tend to be
larger than those in females, which create a deeper voice. The act of
swallowing causes the pharynx and larynx to lift upward, allowing the
pharynx to expand and the epiglottis of the larynx to swing downward,
closing the opening to the trachea. These movements produce a larger area
for food to pass through, while preventing food and beverages from
entering the trachea.
Vocal Cords
Esophagus Pyriform fossa
Trachea
True vocal cord
Vestibular fold
Glottis
Epiglottis
Tongue
The true vocal cords and vestibular folds of the
larynx are viewed looking down from the
laryngopharynx.
Trachea
The trachea (windpipe) extends from the larynx toward the lungs ([link]a).
The trachea is formed by 16 to 20 stacked pieces of cartilage that are
connected by connective tissue. The fibroelastic membrane of the trachea
allows it to stretch and expand slightly during inhalation and exhalation,
whereas the rings of cartilage provide structural support and prevent the
trachea from collapsing.
Trachea
(a) The tracheal tube is formed by stacked, C-shaped pieces of hyaline
cartilage. (b) The layer visible in this cross-section of tracheal wall
tissue between the hyaline cartilage and the lumen of the trachea is the
mucosa, which is composed of pseudostratified ciliated columnar
epithelium that contains goblet cells. LM x 1220. (Micrograph
provided by the Regents of University of Michigan Medical School ©
2012)
Larynx
Trachea
cartilages
Submucosal Pseudostratified
seromucous glands columnar epithelia Cilia =©Lumen
Primary
bronchi
Right lung Left lung Hyaline cartilage
Secondary
bronchi
(a) (b)
Bronchi and Bronchioles
The right and left primary bronchi branch off the trachea towards the right
and left lungs. The primary bronchi further branch into the secondary and
tertiary bronchi. A bronchiole branches from the tertiary bronchi.
Bronchioles, which are about 1 mm in diameter, further branch until they
become the tiny terminal bronchioles, which lead to the structures of gas
exchange. There are more than 1000 terminal bronchioles in each lung. The
muscular walls of the bronchioles do not contain cartilage like those of the
bronchi. However, smooth muscle can change the size of the tubing to
increase or decrease airflow through it.
Respiratory Gas Exchange
The respiratory zone includes structures that are directly involved in gas
exchange. The respiratory zone begins where the terminal bronchioles join
a respiratory bronchiole, the smallest type of bronchiole ({link]), which
then leads to an alveolar duct, opening into a cluster of alveoli.
Respiratory Zone
Terminal bronchiole
Smooth muscle
Deoxygenated blood from
pulmonary artery
Oxygenated blood to
pulmonary vein
Respiratory bronchiole
Alveolar Alveolus
sac
Capillaries
Alveolar
pores
Bronchioles lead to alveolar sacs in the respiratory zone,
where gas exchange occurs.
Alveoli
An alveolar sac is a cluster of many individual alveoli that are responsible
for gas exchange. An alveolus is approximately 200 tm in diameter with
elastic walls that allow the alveolus to stretch during air intake, which
greatly increases the surface area available for gas exchange. Alveoli are
connected to their neighbors by alveolar pores, which help maintain equal
air pressure throughout the alveoli and lung ([link]).
Structures of the Respiratory Zone
(a) The alveolus is responsible for gas exchange. (b) A micrograph
shows the alveolar structures within lung tissue. LM x 178.
(Micrograph provided by the Regents of University of Michigan
Medical School © 2012)
Alveoli Alveolar duct Blood vessels Lumen of bronchiole
Ne, Wee
a re t
+> Alveolar pores f eo cae Lg
| mgr
Capillary 7 See Leal
Respiratory membrane Gx 2 =
Type | alveolar cell ie vhs E oe
\ fs
Macrophage
Alveolus
(gas-filled space)
Type II alveolar cell Alveolar sac
(a) (b)
Note:
Diseases of the...
Respiratory System Disorder: Asthma
Asthma is common condition that affects the lungs in both adults and
children. Approximately 8.2 percent of adults (18.7 million) and 9.4
percent of children (7 million) in the United States suffer from asthma. In
addition, asthma is the most frequent cause of hospitalization in children.
Asthma is a chronic disease characterized by inflammation and fluid
accumulation of the airway, and bronchospasms (that is, constriction of the
bronchioles), which can inhibit air from entering the lungs. In addition,
excessive mucus secretion can occur, which further contributes to blockage
of the airway.
Bronchospasms occur periodically and lead to an “asthma attack.” An
attack may be triggered by environmental factors such as dust, pollen, pet
hair, or dander, changes in the weather, mold, tobacco smoke, and
respiratory infections, or by exercise and stress.
Symptoms of an asthma attack involve coughing, shortness of breath,
wheezing, and tightness of the chest. Symptoms of a severe asthma attack
that requires immediate medical attention would include difficulty
breathing that results in blue (cyanotic) lips or face, confusion, drowsiness,
a rapid pulse, sweating, and severe anxiety. The severity of the condition,
frequency of attacks, and identified triggers influence the type of
medication that an individual may require. Longer-term treatments are
used for those with more severe asthma. Short-term, fast-acting drugs that
are used to treat an asthma attack are typically administered via an inhaler.
For young children or individuals who have difficulty using an inhaler,
asthma medications can be administered via a nebulizer.
Interactive Link Questions
Exercise:
Problem:
Visit this site to learn more about what happens during an asthma
attack. What are the three changes that occur inside the airways during
an asthma attack?
Solution:
Inflammation and the production of a thick mucus; constriction of the
airway muscles, or bronchospasm; and an increased sensitivity to
allergens.
Review Questions
Exercise:
Problem:
Which of the following anatomical structures is at the site of
respiratory gas exchange?
a. pharynx
b. nasal cavity
c. alveoli
d. bronchi
Solution:
C
Exercise:
Problem: What is the function of the conchae in the nasal cavity?
a. increase surface area
b. exchange gases
c. maintain surface tension
d. maintain air pressure
Solution:
A
Exercise:
Problem: Which of the following are structural features of the trachea?
a. C-shaped cartilage
b. smooth muscle fibers
c. cilia
d. all of the above
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
If a person sustains an injury to the epiglottis, what would be the
physiological result?
Solution:
The epiglottis is a region of the larynx that is important during the
swallowing of food or drink. As a person swallows, the pharynx moves
upward and the epiglottis closes over the trachea, preventing food or
drink from entering the trachea. If a person’s epiglottis were injured,
this mechanism would be impaired. As a result, the person may have
problems with food or drink entering the trachea, and possibly, the
lungs. Over time, this may cause infections such as pneumonia to set
in.
References
Bizzintino J, Lee WM, Laing IA, Vang F, Pappas T, Zhang G, Martin AC,
Khoo SK, Cox DW, Geelhoed GC, et al. Association between human
rhinovirus C and severity of acute asthma in children. Eur Respir J
[Internet]. 2010 [cited 2013 Mar 22]; 37(5):1037—1042. Available from:
submit=Go&gca=erj%3B37%2F5%2F 1037 &allch=
Kumar V, Ramzi S, Robbins SL. Robbins Basic Pathology. 7th ed.
Philadelphia (PA): Elsevier Ltd; 2005.
Martin RJ, Kraft M, Chu HW, Berns, EA, Cassell GH. A link between
chronic asthma and chronic infection. J Allergy Clin Immunol [Internet].
2001 [cited 2013 Mar 22]; 107(4):595-601. Available from:
submit=Go&gca=erj%3B37%2F5%2F1037&allch=
Glossary
ala
(plural = alae) small, flaring structure of a nostril that forms the lateral
side of the nares
alar cartilage
cartilage that supports the apex of the nose and helps shape the nares;
it is connected to the septal cartilage and connective tissue of the alae
alveolar duct
small tube that leads from the terminal bronchiole to the respiratory
bronchiole and is the point of attachment for alveoli
alveolar macrophage
immune system cell of the alveolus that removes debris and pathogens
alveolar pore
opening that allows airflow between neighboring alveoli
alveolar sac
cluster of alveoli
alveolus
small, grape-like sac that performs gas exchange in the lungs
apex
tip of the external nose
bronchial tree
collective name for the multiple branches of the bronchi and
bronchioles of the respiratory system
bridge
portion of the external nose that lies in the area of the nasal bones
bronchiole
branch of bronchi that are 1 mm or less in diameter and terminate at
alveolar sacs
bronchus
tube connected to the trachea that branches into many subsidiaries and
provides a passageway for air to enter and leave the lungs
conducting zone
region of the respiratory system that includes the organs and structures
that provide passageways for air and are not directly involved in gas
exchange
cricoid cartilage
portion of the larynx composed of a ring of cartilage with a wide
posterior region and a thinner anterior region; attached to the
esophagus
dorsum nasi
intermediate portion of the external nose that connects the bridge to the
apex and is supported by the nasal bone
epiglottis
leaf-shaped piece of elastic cartilage that is a portion of the larynx that
swings to close the trachea during swallowing
external nose
region of the nose that is easily visible to others
fauces
portion of the posterior oral cavity that connects the oral cavity to the
oropharynx
fibroelastic membrane
specialized membrane that connects the ends of the C-shape cartilage
in the trachea; contains smooth muscle fibers
glottis
opening between the vocal folds through which air passes when
producing speech
laryngeal prominence
region where the two lamina of the thyroid cartilage join, forming a
protrusion known as “Adam’s apple”
laryngopharynx
portion of the pharynx bordered by the oropharynx superiorly and
esophagus and trachea inferiorly; serves as a route for both air and
food
larynx
cartilaginous structure that produces the voice, prevents food and
beverages from entering the trachea, and regulates the volume of air
that enters and leaves the lungs
lingual tonsil
lymphoid tissue located at the base of the tongue
meatus
one of three recesses (superior, middle, and inferior) in the nasal cavity
attached to the conchae that increase the surface area of the nasal
cavity
naris
(plural = nares) opening of the nostrils
nasal bone
bone of the skull that lies under the root and bridge of the nose and is
connected to the frontal and maxillary bones
nasal septum
wall composed of bone and cartilage that separates the left and right
nasal cavities
nasopharynx
portion of the pharynx flanked by the conchae and oropharynx that
serves as an airway
oropharynx
portion of the pharynx flanked by the nasopharynx, oral cavity, and
laryngopharynx that is a passageway for both air and food
palatine tonsil
one of the paired structures composed of lymphoid tissue located
anterior to the uvula at the roof of isthmus of the fauces
paranasal sinus
one of the cavities within the skull that is connected to the conchae that
serve to warm and humidify incoming air, produce mucus, and lighten
the weight of the skull; consists of frontal, maxillary, sphenoidal, and
ethmoidal sinuses
pharyngeal tonsil
structure composed of lymphoid tissue located in the nasopharynx
pharynx
region of the conducting zone that forms a tube of skeletal muscle
lined with respiratory epithelium; located between the nasal conchae
and the esophagus and trachea
philtrum
concave surface of the face that connects the apex of the nose to the
top lip
pulmonary surfactant
substance composed of phospholipids and proteins that reduces the
surface tension of the alveoli; made by type II alveolar cells
respiratory bronchiole
specific type of bronchiole that leads to alveolar sacs
respiratory epithelium
ciliated lining of much of the conducting zone that is specialized to
remove debris and pathogens, and produce mucus
respiratory membrane
alveolar and capillary wall together, which form an air-blood barrier
that facilitates the simple diffusion of gases
respiratory zone
includes structures of the respiratory system that are directly involved
in gas exchange
root
region of the external nose between the eyebrows
thyroid cartilage
largest piece of cartilage that makes up the larynx and consists of two
lamina
trachea
tube composed of cartilaginous rings and supporting tissue that
connects the lung bronchi and the larynx; provides a route for air to
enter and exit the lung
trachealis muscle
smooth muscle located in the fibroelastic membrane of the trachea
true vocal cord
one of the pair of folded, white membranes that have a free inner edge
that oscillates as air passes through to produce sound
type I alveolar cell
squamous epithelial cells that are the major cell type in the alveolar
wall; highly permeable to gases
type II alveolar cell
cuboidal epithelial cells that are the minor cell type in the alveolar
wall; secrete pulmonary surfactant
vestibular fold
part of the folded region of the glottis composed of mucous membrane;
supports the epiglottis during swallowing
Gas Pressure, Volume, and Breathing
By the end of this section, you will be able to:
e Describe the mechanisms that drive breathing
e Discuss how pressure and volume are related
e List the steps involved in breather
e Discuss the physical factors related to breathing
e Discuss factors that can influence the respiratory rate
Breathing can be described as the movement of air into
(inspiration/inhalation) and out of the lungs (expiration/exhalation). The
major mechanism that drive breathing is differences between atmospheric
pressure and the air pressure within the lungs.
Relationship Between Pressure and Volume
Inspiration (or inhalation) and expiration (or exhalation) are dependent on
the differences in pressure between the atmosphere and the lungs. In a gas,
pressure is a force created by the movement of gas molecules that are
confined. For example, a certain number of gas molecules in a two-liter
container has more room than the same number of gas molecules in a one-
liter container ({link]). In this case, the force exerted by the movement of
the gas molecules against the walls of the two-liter container is lower than
the force exerted by the gas molecules in the one-liter container. Therefore,
the pressure is lower in the two-liter container and higher in the one-liter
container. At a constant temperature, changing the volume occupied by the
gas changes the pressure, as does changing the number of gas molecules.
Boyle’s law describes the relationship between volume and pressure in a
gas at a constant temperature. Boyle discovered that the pressure of a gas is
inversely proportional to its volume: If volume increases, pressure
decreases. Likewise, if volume decreases, pressure increases. Pressure and
volume are inversely related (P = k/V). Therefore, the pressure in the one-
liter container (one-half the volume of the two-liter container) would be
twice the pressure in the two-liter container. Boyle’s law is expressed by the
following formula:
Equation:
PV, = PoV2
In this formula, P; represents the initial pressure and V, represents the
initial volume, whereas the final pressure and volume are represented by P>
and V> respectively. If the two- and one-liter containers were connected by
a tube and the volume of one of the containers were changed, then the gases
would move from higher pressure (lower volume) to lower pressure (higher
volume).
Boyle's Law
In a gas, pressure increases as volume
decreases.
Atmospheric pressure is the amount of force that is exerted by gases in the
air surrounding any given surface, such as the body. Atmospheric pressure
can be expressed in millimeters of mercury (mm Hg), which is similar to
the phrase "inches of mercury" used to describe atmospheric pressure on
weather reports. 760 mm Hg is the atmospheric pressure at sea level under
highly specific parameters of latitude and temperature.
How Changes in Volume and Pressure are Accomplished During
Breathing
In addition to the differences in pressures, breathing is also dependent upon
the contraction and relaxation of muscle fibers of both the diaphragm and
thorax. The lungs themselves are passive during breathing, meaning they
are not involved in creating the movement that helps inspiration and
expiration. Contraction and relaxation of the diaphragm and intercostal
muscles (found between the ribs) cause most of the pressure changes that
result in inspiration and expiration. These muscle movements and
subsequent pressure changes cause air to either rush in or be forced out of
the lungs.
During inspiration, the diaphragm and external intercostal muscles contract,
causing the rib cage to expand and move outward, and expanding the
thoracic cavity and lung volume. This creates a lower pressure within the
lung than that of the atmosphere, causing air to be drawn into the lungs.
During expiration, the diaphragm and intercostals relax, causing the thorax
and lungs to recoil. The air pressure within the lungs increases to above the
pressure of the atmosphere, causing air to be forced out of the lungs.
Respiratory Rate
Breathing usually occurs without thought, although at times you can
consciously control it, such as when you swim under water, sing a song, or
blow bubbles. The respiratory rate is the total number of breaths, or
respiratory cycles, that occur each minute. Respiratory rate can be an
important indicator of disease, as the rate may increase or decrease during
an illness or in a disease condition. The respiratory rate is controlled by the
respiratory center located within the brain, which responds primarily to
changes in carbon dioxide, oxygen, and pH levels in the blood.
The normal respiratory rate of a child decreases from birth to adolescence.
A child under 1 year of age has a normal respiratory rate between 30 and 60
breaths per minute, but by the time a child is about 10 years old, the normal
rate is closer to 18 to 30. By adolescence, the normal respiratory rate is
similar to that of adults, 12 to 18 breaths per minute.
Chapter Review
The process of breathing is driven by pressure differences between the
lungs and the atmosphere. Atmospheric pressure is the force exerted by
gases present in the atmosphere. Pressure is determined by the volume of
the space occupied by a gas. Air flows when a pressure gradient is created,
from a space of higher pressure to a space of lower pressure. Boyle’s law
describes the relationship between volume and pressure. A gas is at lower
pressure in a larger volume because the gas molecules have more space to
in which to move. The same quantity of gas in a smaller volume results in
gas molecules crowding together, producing increased pressure.
Pulmonary ventilation consists of the process of inspiration (or inhalation),
where air enters the lungs, and expiration (or exhalation), where air leaves
the lungs. During inspiration, the diaphragm and external intercostal
muscles contract, causing the rib cage to expand and move outward, and
expanding the thoracic cavity and lung volume. This creates a lower
pressure within the lung than that of the atmosphere, causing air to be
drawn into the lungs. During expiration, the diaphragm and intercostals
relax, causing the thorax and lungs to recoil. The air pressure within the
lungs increases to above the pressure of the atmosphere, causing air to be
forced out of the lungs.
Both respiratory rate and depth are controlled by the respiratory centers of
the brain, which are stimulated by factors such as chemical and pH changes
in the blood. A rise in carbon dioxide or a decline in oxygen levels in the
blood stimulates an increase in respiratory rate and depth.
Review Questions
Exercise:
Problem: What are the units for measuring air pressure?
a. mm Hg
b. mm O2
c. Percent Hg
d. Percent O2
Solution:
A
Exercise:
Problem:A decrease in volume leads to a(n) pressure.
a. decrease in
b. equalization of
c. increase in
d. zero
Solution:
C
Exercise:
Problem:
Contraction of the external intercostal muscles causes which of the
following to occur?
a. The diaphragm moves downward.
b. The rib cage is compressed.
c. The thoracic cavity volume decreases.
d. The ribs and sternum move upward.
Solution:
D
Critical Thinking Questions
Exercise:
Problem: Outline the steps involved in quiet breathing.
Solution:
Quiet breathing occurs at rest and without active thought. During quiet
breathing, the diaphragm and external intercostal muscles work at
different extents, depending on the situation. For inspiration, the
diaphragm contracts, causing the diaphragm to flatten and drop
towards the abdominal cavity, helping to expand the thoracic cavity.
The external intercostal muscles contract as well, causing the rib cage
to expand, and the rib cage and sternum to move outward, also
expanding the thoracic cavity. Expansion of the thoracic cavity also
causes the lungs to expand. As a result, the pressure within the lungs
drops below that of the atmosphere, causing air to rush into the lungs.
In contrast, expiration is a passive process. As the diaphragm and
intercostal muscles relax, the lungs and thoracic tissues recoil, and the
volume of the lungs decreases. This causes the pressure within the
lungs to increase above that of the atmosphere, causing air to leave the
lungs.
Exercise:
Problem: What is respiratory rate and how is it controlled?
Solution:
Respiratory rate is defined as the number of breaths taken per minute.
Respiratory rate is controlled by the respiratory center, located in the
brain. Conscious thought can alter the normal respiratory rate through
control by skeletal muscle, although one cannot consciously stop the
rate altogether. A typical resting respiratory rate is about 14 breaths per
minute.
Glossary
alveolar dead space
air space within alveoli that are unable to participate in gas exchange
anatomical dead space
air space present in the airway that never reaches the alveoli and
therefore never participates in gas exchange
apneustic center
network of neurons within the pons that stimulate the neurons in the
dorsal respiratory group; controls the depth of inspiration
atmospheric pressure
amount of force that is exerted by gases in the air surrounding any
given surface
Boyle’s law
relationship between volume and pressure as described by the formula:
PV Lovo
central chemoreceptor
one of the specialized receptors that are located in the brain that sense
changes in hydrogen ion, oxygen, or carbon dioxide concentrations in
the brain
dorsal respiratory group (DRG)
region of the medulla oblongata that stimulates the contraction of the
diaphragm and intercostal muscles to induce inspiration
expiration
(also, exhalation) process that causes the air to leave the lungs
expiratory reserve volume (ERV)
amount of air that can be forcefully exhaled after a normal tidal
exhalation
forced breathing
(also, hyperpnea) mode of breathing that occurs during exercise or by
active thought that requires muscle contraction for both inspiration and
expiration
functional residual capacity (FRC)
sum of ERV and RV, which is the amount of air that remains in the
lungs after a tidal expiration
inspiration
(also, inhalation) process that causes air to enter the lungs
inspiratory capacity (IC)
sum of the TV and IRV, which is the amount of air that can maximally
be inhaled past a tidal expiration
inspiratory reserve volume (IRV)
amount of air that enters the lungs due to deep inhalation past the tidal
volume
intra-alveolar pressure
(intrapulmonary pressure) pressure of the air within the alveoli
intrapleural pressure
pressure of the air within the pleural cavity
peripheral chemoreceptor
one of the specialized receptors located in the aortic arch and carotid
arteries that sense changes in pH, carbon dioxide, or oxygen blood
levels
pneumotaxic center
network of neurons within the pons that inhibit the activity of the
neurons in the dorsal respiratory group; controls rate of breathing
pulmonary ventilation
exchange of gases between the lungs and the atmosphere; breathing
quiet breathing
(also, eupnea) mode of breathing that occurs at rest and does not
require the cognitive thought of the individual
residual volume (RV)
amount of air that remains in the lungs after maximum exhalation
respiratory cycle
one sequence of inspiration and expiration
respiratory rate
total number of breaths taken each minute
respiratory volume
varying amounts of air within the lung at a given time
thoracic wall compliance
ability of the thoracic wall to stretch while under pressure
tidal volume (TV)
amount of air that normally enters the lungs during quiet breathing
total dead space
sum of the anatomical dead space and alveolar dead space
total lung capacity (TLC)
total amount of air that can be held in the lungs; sum of TV, ERV, IRV,
and RV
transpulmonary pressure
pressure difference between the intrapleural and intra-alveolar
pressures
ventral respiratory group (VRG)
region of the medulla oblongata that stimulates the contraction of the
accessory muscles involved in respiration to induce forced inspiration
and expiration
vital capacity (VC)
sum of TV, ERV, and IRV, which is all the volumes that participate in
gas exchange
Gas Exchange
By the end of this section, you will be able to:
e Compare the composition of atmospheric air
e Describe the mechanisms that drive gas exchange
e Discuss the process of external respiration
e Describe the process of internal respiration
The purpose of the respiratory system is to perform gas exchange. Inhaling
provides air to the alveoli for this gas exchange process. At the respiratory
membrane, where the alveolar and capillary walls meet, gases move across
the membranes, with oxygen entering the bloodstream and carbon dioxide
exiting. It is through this mechanism that blood is oxygenated and carbon
dioxide, the waste product of cellular respiration, is removed from the body
via exhaling.
Gas Exchange
In order to understand the mechanisms of gas exchange in the lung, it is
important to understand the underlying principles of gases and their
behavior. In addition to Boyle’s law, several other gas laws help to describe
the behavior of gases.
Gas Laws and Air Composition
Gas molecules exert force on the surfaces with which they are in contact;
this force is called pressure. In natural systems, gases are normally present
as a mixture of different types of molecules. For example, the atmosphere
consists of oxygen, nitrogen, carbon dioxide, and other gaseous molecules,
and this gaseous mixture exerts a certain pressure referred to as atmospheric
pressure ([link]). Partial pressure (P,) is the pressure of a single type of
gas in a mixture of gases. For example, in the atmosphere, oxygen exerts a
partial pressure, and nitrogen exerts another partial pressure, independent of
the partial pressure of oxygen ([link]). Total pressure is the sum of all the
partial pressures of a gaseous mixture.
Partial Pressures of Atmospheric Gases
Percent of total
Gas composition
Nitrogen (N>) 78.6
Oxygen (O>) 20.9
Water (H»O) 0.04
Carbon dioxide (CO>) 0.004
Others 0.0006
Total composition/total
100%
atmospheric pressure
Partial
pressure
(mm Hg)
597.4
158.8
3.0
0.3
0.5
760.0
The partial pressure values are obtained by multiplying by the decimal form
of the percentage (e.g. 0.784) and atmospheric pressure (760 mm Hg). For
example, the partial pressure of oxygen is 0.209 x 760 = 158.8 mm Hg.
Partial and Total Pressures of a Gas
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Oxygen Nitrogen Oxygen + Nitrogen
Pressure Pressure Pressure
159 mm Hg 593 mm Hg 752 mm Hg
Partial pressure is the force exerted by a gas.
The sum of the partial pressures of all the
gases in a mixture equals the total pressure.
Partial pressure is extremely important in predicting the movement of gases.
Recall that gases tend to equalize their pressure in two regions that are
connected. A gas will move from an area where its partial pressure is higher
to an area where its partial pressure is lower. In addition, the greater the
partial pressure difference between the two areas, the more rapid is the
movement of gases.
Solubility of Gases in Liquids
Henry’s law describes the behavior of gases when they come into contact
with a liquid, such as blood. Henry’s law states that the concentration of gas
in a liquid is directly proportional to the solubility and partial pressure of
that gas. The greater the partial pressure of the gas, the greater the number
of gas molecules that will dissolve in the liquid. The concentration of the
gas in a liquid is also dependent on the solubility of the gas in the liquid.
For example, although nitrogen is present in the atmosphere, very little
nitrogen dissolves into the blood, because the solubility of nitrogen in blood
is very low. The exception to this occurs in scuba divers; the composition of
the compressed air that divers breathe causes nitrogen to have a higher
partial pressure than normal, causing it to dissolve in the blood in greater
amounts than normal. Too much nitrogen in the bloodstream results in a
serious condition that can be fatal if not corrected. Gas molecules establish
an equilibrium between those molecules dissolved in liquid and those in air.
The composition of air in the atmosphere and in the alveoli differs. In both
cases, the relative concentration of gases is nitrogen > oxygen > water
vapor > carbon dioxide. The amount of water vapor present in alveolar air
is greater than that in atmospheric air ([link]). Recall that the respiratory
system works to humidify incoming air, thereby causing the air present in
the alveoli to have a greater amount of water vapor than atmospheric air. In
addition, alveolar air contains a greater amount of carbon dioxide and less
oxygen than atmospheric air. This is no surprise, as gas exchange removes
oxygen from and adds carbon dioxide to alveolar air. Both deep and forced
breathing cause the alveolar air composition to be changed more rapidly
than during quiet breathing. As a result, the partial pressures of oxygen and
carbon dioxide change, affecting the diffusion process that moves these
materials across the membrane. This will cause oxygen to enter and carbon
dioxide to leave the blood more quickly.
Composition and Partial Pressures of Alveolar Air
Partial
Percent of total pressure
Gas composition (mm Hg)
Nitrogen (N>) 74.9 969
Oxygen (O>) 13-7. 104
Water (H2O) 6.2 40
Carbon dioxide (CO>) a2 47
Total composition/total 100% 760.0
alveolar pressure
Gas Exchange
Gas exchange occurs at two sites in the body: in the lungs, where oxygen is
picked up and carbon dioxide is released at the respiratory membrane, and
at the tissues, where oxygen is released and carbon dioxide is picked up.
External respiration is the exchange of gases with the external environment,
and occurs in the alveoli of the lungs. Internal respiration is the exchange of
gases with the internal environment, and occurs in the tissues. The actual
exchange of gases occurs due to simple diffusion, because molecular
oxygen and carbon dioxide are small and nonpolar. Energy is not required
to move oxygen or carbon dioxide across membranes. Instead, these gases
follow pressure gradients that allow them to diffuse. The anatomy of the
lung maximizes the diffusion of gases: The respiratory membrane is highly
permeable to gases; the respiratory and blood capillary membranes are very
thin; and there is a large surface area throughout the lungs.
External Respiration
The pulmonary artery carries deoxygenated blood into the lungs from the
heart, where it branches and eventually becomes the capillary network
composed of pulmonary capillaries. These pulmonary capillaries create the
respiratory membrane with the alveoli ((link]). As the blood is pumped
through this capillary network, gas exchange occurs. Although a small
amount of the oxygen is able to dissolve directly into plasma from the
alveoli, most of the oxygen is picked up by erythrocytes (red blood cells)
and binds to a protein called hemoglobin, a process described later in this
chapter. Oxygenated hemoglobin is red, causing the overall appearance of
bright red oxygenated blood, which returns to the heart through the
pulmonary veins. Carbon dioxide is released in the opposite direction of
oxygen, from the blood to the alveoli. Some of the carbon dioxide is
returned on hemoglobin, but can also be dissolved in plasma or is present as
a converted form, also explained in greater detail later in this chapter.
External respiration occurs as a function of partial pressure differences in
oxygen and carbon dioxide between the alveoli and the blood in the
pulmonary capillaries.
External Respiration
Detached from hemoglobin
4 Fused basement membranes Blood plasma
I CO, (dissolved in plasma)
ae
i
L
()
fs
/
O, (dissolved in plasma)
Converted from bicarbonate
In external respiration, oxygen diffuses across
the respiratory membrane from the alveolus to
the capillary, whereas carbon dioxide diffuses
out of the capillary into the alveolus.
The partial pressure of carbon dioxide is also different between the alveolar
air and the blood of the capillary. However, the partial pressure difference is
less than that of oxygen, about 5 mm Hg. The partial pressure of carbon
dioxide in the blood of the capillary is about 45 mm Hg, whereas its partial
pressure in the alveoli is about 40 mm Hg. However, the solubility of
carbon dioxide is much greater than that of oxygen—by a factor of about 20
—in both blood and alveolar fluids. As a result, the relative concentrations
of oxygen and carbon dioxide that diffuse across the respiratory membrane
are similar.
Internal Respiration
Internal respiration is gas exchange that occurs at the level of body tissues
({link]). Similar to external respiration, internal respiration also occurs as
simple diffusion due to a partial pressure gradient. However, the partial
pressure gradients are opposite of those present at the respiratory
membrane. The partial pressure of oxygen in tissues is low because oxygen
is continuously used for cellular respiration. In contrast, the partial pressure
of oxygen in the blood is higher. This creates a pressure gradient that causes
oxygen to dissociate from hemoglobin, diffuse out of the blood, cross the
interstitial space, and enter the tissue. Hemoglobin that has little oxygen
bound to it loses much of its brightness, so that blood returning to the heart
is more burgundy (bluish-red) in color.
Considering that cellular respiration continuously produces carbon dioxide,
the partial pressure of carbon dioxide is lower in the blood than it is in the
tissue, causing carbon dioxide to diffuse out of the tissue, cross the
interstitial fluid, and enter the blood. It is then carried back to the lungs
either bound to hemoglobin, dissolved in plasma, or in a converted form.
By the time blood returns to the heart, the partial pressure of oxygen has
returned to about 40 mm Hg, and the partial pressure of carbon dioxide has
returned to about 45 mm Hg. The blood is then pumped back to the lungs to
be oxygenated once again during external respiration.
Internal Respiration
Blood plasma
—_ ™_ . 5
O, O, (dissolved in plasma)
Oxygen diffuses out of the capillary and into
cells, whereas carbon dioxide diffuses out of
cells and into the capillary.
Note:
Everyday Connection
Hyperbaric Chamber Treatment
A type of device used in some areas of medicine that exploits the behavior
of gases is hyperbaric chamber treatment. A hyperbaric chamber is a unit
that can be sealed and expose a patient to either 100 percent oxygen with
increased pressure or a mixture of gases that includes a higher
concentration of oxygen than normal atmospheric air, also at a higher
partial pressure than the atmosphere. There are two major types of
chambers: monoplace and multiplace. Monoplace chambers are typically
for one patient, and the staff tending to the patient observes the patient
from outside of the chamber ({link]). Some facilities have special
monoplace hyperbaric chambers that allow multiple patients to be treated
at once, usually in a sitting or reclining position, to help ease feelings of
isolation or claustrophobia. Multiplace chambers are large enough for
multiple patients to be treated at one time, and the staff attending these
patients is present inside the chamber. In a multiplace chamber, patients are
often treated with air via a mask or hood, and the chamber is pressurized.
Hyperbaric Chamber
(credit: “komunews”/flickr.com)
Hyperbaric chamber treatment is based on the behavior of gases. As you
recall, gases move from a region of higher partial pressure to a region of
lower partial pressure. In a hyperbaric chamber, the atmospheric pressure is
increased, causing a greater amount of oxygen than normal to diffuse into
the bloodstream of the patient. Hyperbaric chamber therapy is used to treat
a variety of medical problems, such as wound and graft healing, anaerobic
bacterial infections, and carbon monoxide poisoning. Exposure to and
poisoning by carbon monoxide is difficult to reverse, because
hemoglobin’s affinity for carbon monoxide is much stronger than its
affinity for oxygen, causing carbon monoxide to replace oxygen in the
blood. Hyperbaric chamber therapy can treat carbon monoxide poisoning,
because the increased atmospheric pressure causes more oxygen to diffuse
into the bloodstream. At this increased pressure and increased
concentration of oxygen, carbon monoxide is displaced from hemoglobin.
Another example is the treatment of anaerobic bacterial infections, which
are created by bacteria that cannot or prefer not to live in the presence of
oxygen. An increase in blood and tissue levels of oxygen helps to kill the
anaerobic bacteria that are responsible for the infection, as oxygen is toxic
to anaerobic bacteria. For wounds and grafts, the chamber stimulates the
healing process by increasing energy production needed for repair.
Increasing oxygen transport allows cells to ramp up cellular respiration and
thus ATP production, the energy needed to build new structures.
Chapter Review
Each specific gas in a mixture of gases exerts force (its partial pressure)
independently of the other gases in the mixture. Gas molecules move down
a pressure gradient; in other words, gas moves from a region of high
pressure to a region of low pressure. The partial pressure of oxygen is high
in the alveoli and low in the blood of the pulmonary capillaries. As a result,
oxygen diffuses across the respiratory membrane from the alveoli into the
blood. In contrast, the partial pressure of carbon dioxide is high in the
pulmonary capillaries and low in the alveoli. Therefore, carbon dioxide
diffuses across the respiratory membrane from the blood into the alveoli.
The amount of oxygen and carbon dioxide that diffuses across the
respiratory membrane is similar.
External respiration refers to gas exchange that occurs in the alveoli,
whereas internal respiration refers to gas exchange that occurs in the tissue.
Both are driven by partial pressure differences.
Review Questions
Exercise:
Problem:
Gas moves from an area of partial pressure to an area of
partial pressure.
a. low; high
b. low; low
c. high; high
d. high; low
Solution:
D
Exercise:
Problem:
Gas exchange that occurs at the level of the tissues is called
a. external respiration
b. interpulmonary respiration
c. internal respiration
d. pulmonary ventilation
Solution:
C
Exercise:
Problem:
The partial pressure of carbon dioxide is 45 mm Hg in the blood and
40 mm Hg in the alveoli. What happens to the carbon dioxide?
a. It diffuses into the blood.
b. It diffuses into the alveoli.
c. The gradient is too small for carbon dioxide to diffuse.
d. It decomposes into carbon and oxygen.
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
A smoker develops damage to several alveoli that then can no longer
function. How does this affect gas exchange?
Solution:
The damaged alveoli will have insufficient ventilation, causing the
partial pressure of oxygen in the alveoli to decrease. As a result, the
pulmonary capillaries serving these alveoli will constrict, redirecting
blood flow to other alveoli that are receiving sufficient ventilation.
Glossary
Dalton’s law
statement of the principle that a specific gas type in a mixture exerts its
own pressure, as if that specific gas type was not part of a mixture of
gases
external respiration
gas exchange that occurs in the alveoli
Henry’s law
statement of the principle that the concentration of gas in a liquid is
directly proportional to the solubility and partial pressure of that gas
internal respiration
gas exchange that occurs at the level of body tissues
partial pressure
force exerted by each gas in a mixture of gases
total pressure
sum of all the partial pressures of a gaseous mixture
ventilation
movement of air into and out of the lungs; consists of inspiration and
expiration
Transport of Gases
By the end of this section, you will be able to:
e Describe the principles of oxygen transport
¢ Describe the structure of hemoglobin
e Describe the principles of carbon dioxide transport
The other major activity in the lungs is the process of respiration, the
process of gas exchange. The function of respiration is to provide oxygen
for use by body cells during cellular respiration and to eliminate carbon
dioxide, a waste product of cellular respiration, from the body. In order for
the exchange of oxygen and carbon dioxide to occur, both gases must be
transported between the external and internal respiration sites. Both gases
require a specialized transport system for the majority of the gas molecules
to be moved between the lungs and other tissues.
Oxygen Transport in the Blood
The majority of oxygen molecules are carried from the lungs to the body’s
tissues by a specialized transport system, which relies on the erythrocyte—
the red blood cell. Erythrocytes contain hemoglobin, which serves to bind
oxygen molecules to the erythrocyte ([link]). Heme is the portion of
hemoglobin that contains iron, and it is heme that binds oxygen. One
erythrocyte contains four iron ions, and because of this, each erythrocyte is
capable of carrying up to four molecules of oxygen. As oxygen diffuses
across the respiratory membrane from the alveolus to the capillary, it also
diffuses into the red blood cell and is bound by hemoglobin. The following
reversible chemical reaction describes the production of the final product,
oxyhemoglobin (Hb—O;), which is formed when oxygen binds to
hemoglobin. Oxyhemoglobin is a bright red-colored molecule that
contributes to the bright red color of oxygenated blood.
Equation:
Hb + O, © Hb = O,
Erythrocyte and Hemoglobin
Hemoglobin consists of four
subunits, each of which
contains one molecule of iron.
Function of Hemoglobin
Hemoglobin is composed of subunits, a protein structure that is referred to
as a quaternary structure. Each of the four subunits that make up
hemoglobin is arranged in a ring-like fashion, with an iron atom covalently
bound to the heme in the center of each subunit. When all four heme sites
are occupied, the hemoglobin is said to be saturated. Hemoglobin saturation
of 100 percent means that every heme unit in all of the erythrocytes of the
body is bound to oxygen. In a healthy individual with normal hemoglobin
levels, hemoglobin saturation generally ranges from 95 percent to 99
percent.
Carbon Dioxide Transport in the Blood
Carbon dioxide is transported by three major mechanisms. The first
mechanism of carbon dioxide transport is by blood plasma, as some carbon
dioxide molecules dissolve in the blood. The second mechanism is transport
in the form of bicarbonate (HCO3 ), which also dissolves in plasma. The
third mechanism of carbon dioxide transport is similar to the transport of
oxygen by erythrocytes ((Llink]).
Carbon Dioxide Transport
(a) CO, carried in RBC
(b) HCO, dissolved in
plasma as carbonic
acid
(c) CO, dissolved in
plasma
Carbon dioxide is transported by three different
methods: (a) in erythrocytes; (b) after forming
carbonic acid (H»CO3 ), which is dissolved in
plasma; (c) and in plasma.
Dissolved Carbon Dioxide
Although carbon dioxide is not considered to be highly soluble in blood, a
small fraction—about 7 to 10 percent—of the carbon dioxide that diffuses
into the blood from the tissues dissolves in plasma. The dissolved carbon
dioxide then travels in the bloodstream and when the blood reaches the
pulmonary capillaries, the dissolved carbon dioxide diffuses across the
respiratory membrane into the alveoli, where it is then exhaled during
pulmonary ventilation.
Bicarbonate Buffer
A large fraction—about 70 percent—of the carbon dioxide molecules that
diffuse into the blood is transported to the lungs as bicarbonate. Most
bicarbonate is produced in erythrocytes after carbon dioxide diffuses into
the capillaries, and subsequently into red blood cells. Carbonic anhydrase
(CA) causes carbon dioxide and water to form carbonic acid (H2COs3),
which dissociates into two ions: bicarbonate (HCO3_) and hydrogen (H”).
The following formula depicts this reaction:
Equation:
CO, + H,0 @ H,CO; + Ht + HCO3_
At the pulmonary capillaries, the chemical reaction that produced
bicarbonate (shown above) is reversed, and carbon dioxide and water are
the products. Hydrogen ions and bicarbonate ions join to form carbonic
acid, which is converted into carbon dioxide and water by carbonic
anhydrase. Carbon dioxide diffuses out of the erythrocytes and into the
plasma, where it can further diffuse across the respiratory membrane into
the alveoli to be exhaled during pulmonary ventilation.
Carbaminohemoglobin
About 20 percent of carbon dioxide is bound by hemoglobin and is
transported to the lungs. Carbon dioxide does not bind to iron as oxygen
does; instead, carbon dioxide binds amino acids on the globin portions of
hemoglobin to form carbaminohemoglobin, which forms when
hemoglobin and carbon dioxide bind. When hemoglobin is not transporting
oxygen, it tends to have a bluish-purple tone to it, creating the darker
maroon color typical of deoxygenated blood. The following formula depicts
this reversible reaction:
Equation:
Similar to the transport of oxygen by heme, the binding and dissociation of
carbon dioxide to and from hemoglobin is dependent on the partial pressure
of carbon dioxide. Because carbon dioxide is released from the lungs, blood
that leaves the lungs and reaches body tissues has a lower partial pressure of
carbon dioxide than is found in the tissues. As a result, carbon dioxide
leaves the tissues because of its higher partial pressure, enters the blood,
and then moves into red blood cells, binding to hemoglobin. In contrast, in
the pulmonary capillaries, the partial pressure of carbon dioxide is high
compared to within the alveoli. As a result, carbon dioxide dissociates
readily from hemoglobin and diffuses across the respiratory membrane into
the air.
Chapter Review
Oxygen is primarily transported through the blood by erythrocytes. These
cells contain a protein molecule called hemoglobin, which is composed of
four subunits with a ring-like structure. Each subunit contains one atom of
iron bound to a molecule of heme. Heme binds oxygen so that each
hemoglobin molecule can bind up to four oxygen molecules. When all of
the heme units in the blood are bound to oxygen, hemoglobin is considered
to be saturated.
Carbon dioxide is transported in blood by three different mechanisms: as
dissolved carbon dioxide, as bicarbonate, or as carbaminohemoglobin. A
small portion of carbon dioxide remains. The largest amount of transported
carbon dioxide is as bicarbonate, formed in erythrocytes. For this
conversion, carbon dioxide is combined with water with the aid of an
enzyme called carbonic anhydrase. This combination forms carbonic acid,
which spontaneously dissociates into bicarbonate and hydrogen ions. As
bicarbonate builds up in erythrocytes, it is moved across the membrane into
the plasma. At the pulmonary capillaries, bicarbonate re-enters erythrocytes
and the reaction with carbonic anhydrase is reversed, recreating carbon
dioxide and water. Carbon dioxide then diffuses out of the erythrocyte and
across the respiratory membrane into the air. An intermediate amount of
carbon dioxide binds directly to hemoglobin to form carbaminohemoglobin.
Interactive Link Questions
Exercise:
Problem:
Watch this video to see the transport of oxygen from the lungs to the
tissues. Why is oxygenated blood bright red, whereas deoxygenated
blood tends to be more of a purple color?
Solution:
When oxygen binds to the hemoglobin molecule, oxyhemoglobin is
created, which has a red color to it. Hemoglobin that is not bound to
oxygen tends to be more of a blue—purple color. Oxygenated blood
traveling through the systemic arteries has large amounts of
oxyhemoglobin. As blood passes through the tissues, much of the
oxygen is released into systemic capillaries. The deoxygenated blood
returning through the systemic veins, therefore, contains much smaller
amounts of oxyhemoglobin. The more oxyhemoglobin that is present
in the blood, the redder the fluid will be. As a result, oxygenated blood
will be much redder in color than deoxygenated blood.
Review Questions
Exercise:
Problem:
Oxyhemoglobin forms by a chemical reaction between which of the
following?
a. hemoglobin and carbon dioxide
b. carbonic anhydrase and carbon dioxide
c. hemoglobin and oxygen
d. carbonic anhydrase and oxygen
Solution:
C
Exercise:
Problem:
In what form is the majority of carbon dioxide transported in the
blood?
a. Bicarbonate ion
b. Carbaminohemoglobin
c. Carbonic acid
d. Carbonic anhydrase
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
Describe the relationship between the partial pressure of oxygen and
the binding of oxygen to hemoglobin.
Solution:
As the partial pressure of oxygen increases, the number of oxygen
molecules bound by hemoglobin increases, thereby increasing the
saturation of hemoglobin.
Exercise:
Problem:
Describe three ways in which carbon dioxide can be transported.
Solution:
Carbon dioxide can be transported by three mechanisms: dissolved in
plasma, as bicarbonate, or as carbaminohemoglobin. Dissolved in
plasma, carbon dioxide molecules simply diffuse into the blood from
the tissues. Bicarbonate is created by a chemical reaction that occurs
mostly in erythrocytes, joining carbon dioxide and water by carbonic
anhydrase, producing carbonic acid, which breaks down into
bicarbonate and hydrogen ions. Carbaminohemoglobin is the bound
form of hemoglobin and carbon dioxide.
Glossary
Bohr effect
relationship between blood pH and oxygen dissociation from
hemoglobin
carbaminohemoglobin
bound form of hemoglobin and carbon dioxide
carbonic anhydrase (CA)
enzyme that catalyzes the reaction that causes carbon dioxide and
water to form carbonic acid
chloride shift
facilitated diffusion that exchanges bicarbonate (HCO3_) with chloride
(CI) ions
Haldane effect
relationship between the partial pressure of oxygen and the affinity of
hemoglobin for carbon dioxide
oxyhemoglobin
(Hb—O,) bound form of hemoglobin and oxygen
oxygen—hemoglobin dissociation curve
graph that describes the relationship of partial pressure to the binding
and disassociation of oxygen to and from heme
Endocrine System
By the end of this section, you will be able to:
e List the different types of hormones and explain their roles in
maintaining homeostasis
e Explain how hormones work
e Explain how hormone production is regulated
e Describe the role of different glands in the endocrine system
e Explain how the different glands work together to maintain
homeostasis
The endocrine system produces hormones that function to control and
regulate many different body processes. The endocrine system coordinates
with the nervous system to control the functions of the other organ systems.
Cells of the endocrine system produce molecular signals called hormones.
These cells may compose endocrine glands, may be tissues or may be
located in organs or tissues that have functions in addition to hormone
production. Hormones circulate throughout the body and stimulate a
response in cells that have receptors able to bind with them. The changes
brought about in the receiving cells affect the functioning of the organ
targeted by the hormone. Many of the hormones are secreted in response to
signals from the nervous system, thus the two systems act in concert to
effect changes in the body.
Hormones
Maintaining homeostasis within the body requires the coordination of many
different systems and organs. One mechanism of communication between
neighboring cells, and between cells and tissues in distant parts of the body,
occurs through the release of chemicals called hormones. Hormones are
released into body fluids, usually blood, which carries them to their target
cells where they elicit a response. The cells that secrete hormones are often
located in specific organs, called endocrine glands, and the cells, tissues,
and organs that secrete hormones make up the endocrine system. Examples
of endocrine organs include the pancreas, which produces the hormones
insulin and glucagon to regulate blood-glucose levels, the adrenal glands,
which produce hormones such as epinephrine and norepinephrine that
regulate responses to stress, and the thyroid gland, which produces thyroid
hormones that regulate metabolic rates.
The endocrine glands differ from the exocrine glands. Exocrine glands
secrete chemicals through ducts that lead outside the gland (not to the
blood). For example, sweat produced by sweat glands is released into ducts
that carry sweat to the surface of the skin. The pancreas has both endocrine
and exocrine functions because besides releasing hormones into the blood.
It also produces digestive juices, which are carried by ducts into the small
intestine.
Note:
Career in Action
Endocrinologist
An endocrinologist is a medical doctor who specializes in treating
endocrine disorders. An endocrine surgeon specializes in the surgical
treatment of endocrine diseases and glands. Some of the diseases that are
managed by endocrinologists include disorders of the pancreas (diabetes
mellitus), disorders of the pituitary (gigantism, acromegaly, and pituitary
dwarfism), disorders of the thyroid gland (goiter and Graves’ disease), and
disorders of the adrenal glands (Cushing’s disease and Addison’s disease).
Endocrinologists are required to assess patients and diagnose endocrine
disorders through extensive use of laboratory tests. Many endocrine
diseases are diagnosed using tests that stimulate or suppress endocrine
organ functioning. Blood samples are then drawn to determine the effect of
stimulating or suppressing an endocrine organ on the production of
hormones. For example, to diagnose diabetes mellitus, patients are required
to fast for 12 to 24 hours. They are then given a sugary drink, which
stimulates the pancreas to produce insulin to decrease blood-glucose levels.
A blood sample is taken one to two hours after the sugar drink is
consumed. If the pancreas is functioning properly, the blood-glucose level
will be within a normal range. Another example is the A1C test, which can
be performed during blood screening. The A1C test measures average
blood-glucose levels over the past two to three months. The A1C test is an
indicator of how well blood glucose is being managed over a long time.
Once a disease such as diabetes has been diagnosed, endocrinologists can
prescribe lifestyle changes and medications to treat the disease. Some cases
of diabetes mellitus can be managed by exercise, weight loss, and a healthy
diet; in other cases, medications may be required to enhance insulin’s
production or effect. If the disease cannot be controlled by these means, the
endocrinologist may prescribe insulin injections.
In addition to clinical practice, endocrinologists may also be involved in
primary research and development activities. For example, ongoing islet
transplant research is investigating how healthy pancreas islet cells may be
transplanted into diabetic patients. Successful islet transplants may allow
patients to stop taking insulin injections.
How Hormones Work
Hormones cause changes in target cells by binding to specific cell-surface
or intracellular hormone receptors, molecules embedded in the cell
membrane or floating in the cytoplasm with a binding site that matches a
binding site on the hormone molecule. In this way, even though hormones
circulate throughout the body and come into contact with many different
cell types, they only affect cells that possess the necessary receptors.
Receptors for a specific hormone may be found on or in many different
cells or may be limited to a small number of specialized cells. For example,
thyroid hormones act on many different tissue types, stimulating metabolic
activity throughout the body. Cells can have many receptors for the same
hormone but often also possess receptors for different types of hormones.
The number of receptors that respond to a hormone determines the cell’s
sensitivity to that hormone, and the resulting cellular response.
Additionally, the number of receptors available to respond to a hormone can
change over time, resulting in increased or decreased cell sensitivity.
Endocrine Glands
The endocrine glands secrete hormones into the surrounding interstitial
fluid; those hormones then diffuse into blood and are carried to various
organs and tissues within the body. The endocrine glands include the
pituitary, thyroid, parathyroid, adrenal glands, gonads, pineal, and pancreas.
The pituitary gland is located at the base of the brain ({link]a). It is
attached to the hypothalamus. The posterior lobe stores and releases
oxytocin and antidiuretic hormone (ADH) produced by the hypothalamus.
The anterior lobe responds to hormones produced by the hypothalamus by
producing its own hormones, most of which regulate other hormone-
producing glands.
Pineal gland
Cerebellum
Pituitary gland
Pons
Medulla oblongata __ |
\
Spinal cord
(a) (b)
Adrenal gland
Gall bladder
Stomach
Bile duct
Duodenum
Pancreas
Kidney Pancreatic duct
(c) (d)
(a) The pituitary gland sits at the base of the brain, just above
the brain stem. (b) The parathyroid glands are located on the
posterior of the thyroid gland. (c) The adrenal glands are on top
of the kidneys. d) The pancreas is found between the stomach
and the small intestine. (credit: modification of work by NCI,
NIH)
The anterior pituitary produces six hormones: growth hormone, prolactin,
thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-
stimulating hormone (FSH), and luteinizing hormone (LH). Growth
hormone stimulates cellular activities like protein synthesis that promote
growth. Prolactin stimulates the production of milk by the mammary
glands. The other hormones produced by the anterior pituitary regulate the
production of hormones by other endocrine tissues ({link]). The posterior
pituitary is significantly different in structure from the anterior pituitary. It
is a part of the brain, extending down from the hypothalamus, and contains
mostly nerve fibers that extend from the hypothalamus to the posterior
pituitary.
The thyroid gland is located in the neck, just below the larynx and in front
of the trachea ({link |b). It is a butterfly-shaped gland with two lobes that are
connected. The thyroid follicle cells synthesize the hormone thyroxine,
which is also known as Ty because it contains four atoms of iodine, and
triiodothyronine, also known as T3 because it contains three atoms of
iodine. T3 and Ty are released by the thyroid in response to thyroid-
stimulating hormone produced by the anterior pituitary, and both T3 and T,
have the effect of stimulating metabolic activity in the body and increasing
energy use. A third hormone, calcitonin, is also produced by the thyroid.
Calcitonin is released in response to rising calcium ion concentrations in the
blood and has the effect of reducing those levels.
Most people have four parathyroid glands; however, the number can vary
from two to six. These glands are located on the posterior surface of the
thyroid gland ([link]b).
The parathyroid glands produce parathyroid hormone. Parathyroid hormone
increases blood calcium concentrations when calcium ion levels fall below
normal.
The adrenal glands are located on top of each kidney ((link]c). The adrenal
glands consist of an outer adrenal cortex and an inner adrenal medulla.
These regions secrete different hormones.
The adrenal cortex produces mineralocorticoids, glucocorticoids, and
gonadocorticoids (principally androgens). The main mineralocorticoid is
aldosterone, which regulates the concentration of ions in urine, sweat, and
saliva. Aldosterone release from the adrenal cortex is stimulated by a
decrease in blood concentrations of sodium ions, blood volume, or blood
pressure, or by an increase in blood potassium levels. The glucocorticoids
maintain proper blood-glucose levels between meals. They also control a
response to stress by increasing glucose synthesis from fats and proteins
and interact with epinephrine to cause vasoconstriction. Androgens are sex
hormones that are produced in small amounts by the adrenal cortex. They
do not normally affect sexual characteristics and may supplement sex
hormones released from the gonads. The adrenal medulla contains two
types of secretory cells: one that produces epinephrine (adrenaline) and
another that produces norepinephrine (noradrenaline). Epinephrine and
norepinephrine cause immediate, short-term changes in response to
stressors, inducing the so-called fight-or-flight response. The responses
include increased heart rate, breathing rate, cardiac muscle contractions,
and blood-glucose levels. They also accelerate the breakdown of glucose in
skeletal muscles and stored fats in adipose tissue, and redirect blood flow
toward skeletal muscles and away from skin and viscera. The release of
epinephrine and norepinephrine is stimulated by neural impulses from the
sympathetic nervous system that originate from the hypothalamus.
The pancreas is an elongate organ located between the stomach and the
proximal portion of the small intestine ((link]d). It contains both exocrine
cells that excrete digestive enzymes and endocrine cells that release
hormones.
The endocrine cells of the pancreas form clusters called pancreatic islets or
the islets of Langerhans. Among the cell types in each pancreatic islet are
the alpha cells, which produce the hormone glucagon, and the beta cells,
which produce the hormone insulin. These hormones regulate blood-
glucose levels. Alpha cells release glucagon as blood-glucose levels
decline. When blood-glucose levels rise, beta cells release insulin.
Glucagon causes the release of glucose to the blood from the liver, and
insulin facilitates the uptake of glucose by the body’s cells.
The gonads(testes in the male and ovaries in the female)produce steroid
hormones. The testes produce androgens, testosterone being the most
prominent, which allow for the development of secondary sex
characteristics and the production of sperm cells. The ovaries produce
estrogen and progesterone, which cause secondary sex characteristics,
regulate production of eggs, control pregnancy, and prepare the body for
childbirth.
The kidneys also possess endocrine function. Erythropoietin (EPO) is
released by kidneys in response to low blood oxygen levels. EPO triggers
an increase in the rate of production of red blood cells in the red bone
marrow. EPO has been used by athletes (e.g. cyclists) to improve
performance because more RBCs mean more oxygen can be transported.
However, EPO doping has its risks, because it thickens the blood and
increases strain on the heart; it also increases the risk of blood clots and
therefore heart attacks and stroke.
Endocrine Glands and Their Associated Hormones
Endocrine Associated
Gland Hormones Effect
Pituitary promotes growth of body
growth hormone
(anterior) tissues
Endocrine Glands and Their Associated Hormones
Endocrine
Gland
Pituitary
(posterior)
Thyroid
Parathyroid
Associated
Hormones
prolactin
thyroid-stimulating
hormone
adrenocorticotropic
hormone
follicle-stimulating
hormone
luteinizing
hormone
antidiuretic
hormone
oxytocin
thyroxine,
triiodothyronine
calcitonin
parathyroid
hormone
Effect
promotes milk production
stimulates thyroid hormone
release
stimulates hormone release
by adrenal cortex
stimulates gamete
production
stimulates androgen
production by gonads in
males; stimulates ovulation
and production of estrogen
and progesterone in females
stimulates water
reabsorption by kidneys
stimulates uterine
contractions during
childbirth
stimulate metabolism
reduces blood Ca2* levels
increases blood Ca2* levels
Endocrine Glands and Their Associated Hormones
Endocrine Associated
Gland Hormones Effect
aldosterone increases blood Na’ levels
Adrenal
cortisol, .
(cortex) ; increase blood-glucose
corticosterone,
levels
cortisone
Adrenal epinephrine, stimulate fight-or-flight
(medulla) norepinephrine response
ae reduces blood-glucose
insulin
levels
Pancreas
increases blood-glucose
glucagon
levels
Regulation of Hormone Production
Hormone production and release are primarily controlled by negative
feedback, as described in the discussion on homeostasis. In this way, the
concentration of hormones in blood is maintained within a narrow range.
For example, the anterior pituitary signals the thyroid to release thyroid
hormones. Increasing levels of these hormones in the blood then give
feedback to the hypothalamus and anterior pituitary to inhibit further
signaling to the thyroid gland ((link]).
Note:
Art Connection
Thyroid System
opin-releasing
e (TRH)
d-stimulating
one (TSH)
The anterior pituitary stimulates the
thyroid gland to release thyroid hormones
T3 and Ty. Increasing levels of these
hormones in the blood result in feedback
to the hypothalamus and anterior
pituitary to inhibit further signaling to the
thyroid gland. (credit: modification of
work by Mikael Haggstr6m)
Section Summary
Hormones cause cellular changes by binding to receptors on or in target
cells. The number of receptors on a target cell can increase or decrease in
response to hormone activity.
Hormone levels are primarily controlled through negative feedback, in
which rising levels of a hormone inhibit its further release.
The pituitary gland is located at the base of the brain. The anterior pituitary
receives signals from the hypothalamus and produces six hormones. The
posterior pituitary is an extension of the brain and releases hormones
(antidiuretic hormone and oxytocin) produced by the hypothalamus. The
thyroid gland is located in the neck and is composed of two lobes. The
thyroid produces the hormones thyroxine and triiodothyronine. The thyroid
also produces calcitonin. The parathyroid glands lie on the posterior surface
of the thyroid gland and produce parathyroid hormone.
The adrenal glands are located on top of the kidneys and consist of the
adrenal cortex and adrenal medulla. The adrenal cortex produces the
glucocorticoids, mineralocorticoids, and gonadocorticoids. The adrenal
medulla is the inner part of the adrenal gland and produces epinephrine and
norepinephrine.
The pancreas lies in the abdomen between the stomach and the small
intestine. Clusters of endocrine cells in the pancreas form the islets of
Langerhans, which contain alpha cells that release glucagon and beta cells
that release insulin. The kidneys produce erythropoietin. The gonads
produce steroid hormones, including testosterone in males and estrogen and
progesterone in females.
Art Connections
Exercise:
Problem:
Goiter, a disease caused by iodine deficiency, results in the inability of
the thyroid gland to form T3 and T,. The body typically attempts to
compensate by producing greater amounts of TSH. Which of the
following symptoms would you expect goiter to cause?
a. Hypothyroidism, resulting in weight gain, cold sensitivity, and
reduced mental activity.
b. Hyperthyroidism, resulting in weight loss, profuse sweating and
increased heart rate.
c. Hyperthyroidism, resulting in weight gain, cold sensitivity, and
reduced mental activity.
d. Hypothyroidism, resulting in weight loss, profuse sweating and
increased heart rate.
Solution:
[link]A
Review Questions
Exercise:
Problem:
Most of the hormones produced by the anterior pituitary perform what
function?
a. regulate growth
b. regulate the sleep cycle
c. regulate production of other hormones
d. regulate blood volume and blood pressure
Solution:
C
Exercise:
Problem: What is the function of the hormone erythropoietin?
a. stimulates production of red blood cells
b. stimulates muscle growth
c. causes the fight-or-flight response
d. causes testosterone production
Solution:
A
Exercise:
Problem: Which endocrine glands are associated with the kidneys?
a. thyroid glands
b. pituitary glands
c. adrenal glands
d. gonads
Solution:
C
Free Response
Exercise:
Problem:
What is a similarity and a difference between an exocrine gland and an
endocrine gland?
Solution:
The cells of both exocrine and endocrine glands produce a product that
will be secreted by the gland. An exocrine gland has a duct and
secretes its product to the outside of the gland, not into the
bloodstream. An endocrine gland secretes its product into the
bloodstream and does not use a duct.
Exercise:
Problem:
Many hormone systems regulate body functions through opposing
hormone actions. Describe how opposing hormone actions regulate
blood-glucose levels?
Solution:
Blood-glucose levels are regulated by hormones produced by the
pancreas: insulin and glucagon. When blood-glucose levels are
increasing, the pancreas releases insulin, which stimulates uptake of
glucose by cells. When blood-glucose levels are decreasing, the
pancreas releases glucagon, which stimulates the release of stored
glucose by the liver to the bloodstream.
Glossary
adrenal gland
the endocrine gland associated with the kidneys
down-regulation
a decrease in the number of hormone receptors in response to
increased hormone levels
endocrine gland
the gland that secretes hormones into the surrounding interstitial fluid,
which then diffuse into blood and are carried to various organs and
tissues within the body
exocrine gland
the gland that secretes chemicals through ducts that lead to skin
surfaces, body cavities, and organ cavities.
hormone
a chemical released by cells in one area of the body that affects cells in
other parts of the body
intracellular hormone receptor
a hormone receptor in the cytoplasm or nucleus of a cell
pancreas
the organ located between the stomach and the small intestine that
contains exocrine and endocrine cells
parathyroid gland
the gland located on the surface of the thyroid that produces
parathyroid hormone
pituitary gland
the endocrine gland located at the base of the brain composed of an
anterior and posterior region; also called hypophysis
thymus
the gland located behind the sternum that produces thymosin hormones
that contribute to the development of the immune system
thyroid gland
an endocrine gland located in the neck that produces thyroid hormones
thyroxine and triiodothyronine
up-regulation
an increase in the number of hormone receptors in response to
increased hormone levels
Introduction to the Urinary System
class="introduction"
Sewage Treatment Plant
(credit:
“eutrophication&hypoxia’/flickr.com
Note:
Chapter Objectives
After studying this chapter, you will be able to:
e Describe the composition of urine
e Label structures of the urinary system
e Characterize the roles of each of the parts of the urinary system
e Trace the flow of blood through the kidney
¢ Outline how blood is filtered in the kidney nephron
e List some of the solutes filtered, secreted, and reabsorbed in different
parts of the nephron
The urinary system has roles you may be well aware of: cleansing the blood
and ridding the body of wastes probably come to mind. However, there are
additional, equally important functions played by the system. Take for
example, regulation of pH, a function shared with the lungs and the buffers
in the blood. Additionally, the regulation of blood pressure is a role shared
with the heart and blood vessels. What about regulating the concentration of
solutes in the blood? Did you know that the kidney is important in
determining the concentration of red blood cells? Eighty-five percent of the
erythropoietin (EPO) produced to stimulate red blood cell production is
produced in the kidneys. The kidneys also perform the final synthesis step
of vitamin D production.
If the kidneys fail, these functions are compromised or lost altogether, with
devastating effects on homeostasis. The affected individual might
experience weakness, lethargy, shortness of breath, anemia, widespread
edema (swelling), metabolic acidosis, rising potassium levels, heart
arrhythmias, and more. Each of these functions is vital to your well-being
and survival. The urinary system, controlled by the nervous system, also
stores urine until a convenient time for disposal and then provides the
anatomical structures to transport this waste liquid to the outside of the
body. Failure of nervous control or the anatomical structures leading to a
loss of control of urination results in a condition called incontinence.
This chapter will help you to understand the anatomy of the urinary system
and how it enables the physiologic functions critical to homeostasis. It is
best to think of the kidney as a regulator of plasma makeup rather than
simply a urine producer. As you read each section, ask yourself this
question: “What happens if this does not work?” This question will help
you to understand how the urinary system maintains homeostasis and
affects all the other systems of the body and the quality of one’s life.
Note:
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Watch this video from the Howard Hughes Medical Institute for an
introduction to the urinary system.
Urinary System Anatomy and Function
By the end of this section, you will be able to:
e Identify the following structures of the urinary system and describe
their function: kidney, ureters, bladder, and urethra
e Describe the structure of the kidneys and the functions of the parts of
the kidney
¢ Describe how the nephron is the functional unit of the kidney and
explain how it actively filters blood and generates urine
e Detail the three steps in the formation of urine: glomerular filtration,
tubular reabsorption, and tubular secretion
Anatomy of the Urinary System
The kidneys, illustrated in [link], are a pair of bean-shaped structures that
are located just below and behind the liver in the abdominal cavity. The
adrenal glands sit on top of each kidney and function as a component of the
endocrine system. Kidneys filter blood and purify it. All the blood in the
human body is filtered many times a day by the kidneys; these organs use
up almost 25 percent of the oxygen absorbed through the lungs to perform
this function. Oxygen allows the kidney cells to efficiently manufacture
chemical energy in the form of ATP through aerobic respiration. The filtrate
coming out of the kidneys is called urine. Urine is carried from the kidneys
to the urinary bladder via the ureters, which are approximately 30 cm
long. As urine passes through the ureters, it does not passively drain into the
bladder but rather is propelled by waves of peristalsis (smooth muscle
contractions). The bladder collects urine from both ureters . During late
pregnancy, its capacity (typically several hundred milliliters) is reduced due
to compression by the enlarging uterus, resulting in increased frequency of
urination. The urethra transports urine from the bladder to the outside of
the body for disposal. The urethra is the only urologic organ that shows any
significant anatomic difference between males and females; all other urine
transport structures are identical. In females, the urethra is relatively short
length, about 4 cm, and is less of a barrier to fecal bacteria than the longer
male urethra (approximately 20 cm). This length difference is the best
explanation for the greater incidence of urinary tract infections (UTIs) in
women. The urethra in males also has a reproductive function, as it
transports semen (sperm and accessory fluids).
Renal artery
Adrenal
gland
Kidney Renal vein
Ureter
Renal
pelvis
Bladder
Urethra
Kidneys filter the blood,
producing urine that is stored in
the bladder prior to elimination
through the urethra. (credit:
modification of work by NCI)
Kidney Structure
Internally, the kidney has three regions—an outer cortex, a medulla in the
middle, and the renal pelvis in the region called the hilum of the kidney.
The hilum is the concave part of the bean-shape where blood vessels and
nerves enter and exit the kidney; it is also the point of exit for the ureters.
The renal cortex is granular due to the presence of renal corpuscles;
nephron tubules can be found throughout the renal cortex and renal
pyramids, the multiple tissue masses that make up the majority of the renal
medulla. There are, on average, eight renal pyramids in each kidney. Urine
that is produced by the nephrons travels into the renal pelvis and then into
the ureters, which carry the urine to the bladder.
Note:
Art Connection
Capillaries
Arteriole
Venule
' Renal pyramid
Renal vein
Renal artery
Renal pelvis
Major calyx
Minor calyx
Medulla
Ureter
Renal fascia
and capsule
Cortex
The internal structure of the kidney is shown.
(credit: modification of work by NCI)
Because the kidney filters blood, its network of blood vessels is an
important component of its structure and function. The arteries, veins, and
nerves that supply the kidney enter and exit at the renal hilum. Renal blood
supply starts with the branching of the aorta into the renal arteries and
ends with the exiting of the renal veins to join the inferior vena cava,
which transports blood back to the right atrium of the heart. The renal
arteries split multiple times to form other blood vessels before branching
into numerous afferent arterioles, and then enter the capillaries supplying
the nephrons.
As mentioned previously, the functional unit of the kidney is the nephron,
illustrated in [link]. Each kidney is made up of over one million nephrons
that dot the renal cortex. A nephron consists of three parts—a renal
corpuscle, a renal tubule, and the associated capillary network.
Note:
Art Connection
Proximal convoluted
Nephron tubule
[2
} Medulla
Peritubular
aria Distal
capillaries
convoluted
tubule
Efferent
arteriole
Bowman's
capsule
Afferent
arteriole
The nephron is the functional unit of the kidney. The
glomerulus and convoluted tubules are located in the
kidney cortex, while collecting ducts are located in the
pyramids of the medulla. (credit: modification of work by
NIDDK)
Renal Corpuscle
The renal corpuscle, located in the renal cortex, is made up of a network of
capillaries known as the glomerulus and the capsule, a cup-shaped chamber
that surrounds it, called the glomerular or Bowman's capsule.
Renal Tubule
The renal tubule is a long and convoluted structure that emerges from the
glomerulus and can be divided into three parts based on function. The first
part is called the proximal convoluted tubule (PCT) due to its proximity
to the glomerulus. The second part is called the loop of Henle, because it
forms a loop (with descending and ascending limbs). The third part of the
renal tubule is called the distal convoluted tubule (DCT). The DCT, which
is the last part of the nephron, connects and empties its contents into
collecting ducts. The urine will ultimately move into the renal pelvis and
then into the ureters.
Capillary Network within the Nephron
The capillary network that originates from the renal arteries supplies the
nephron with blood that needs to be filtered. The branch that enters the
glomerulus is called the afferent arteriole. The branch that exits the
glomerulus is called the efferent arteriole. Within the glomerulus, the
network of capillaries is called the glomerular capillary bed. Once the
efferent arteriole exits the glomerulus, it forms the peritubular capillary
network, which surrounds and interacts with parts of the renal tubule.
Note:
Link to Learning
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Go to this website to see another section of the kidney and to explore an
animation of the workings of nephrons.
Kidney Function and Physiology
Kidneys filter blood in a three-step process. First, the nephrons filter blood
that runs through the capillary network in the glomerulus. Almost all
solutes, except for proteins, are filtered out into the glomerulus by a process
called glomerular filtration. Second, the filtrate is collected in the renal
tubules. Most of the solutes get reabsorbed in the PCT by a process called
tubular reabsorption. In the loop of Henle, the filtrate continues to
exchange solutes and water with the peritubular capillary network. Water is
also reabsorbed during this step. Then, additional solutes and wastes are
secreted into the kidney tubules during tubular secretion, which is, in
essence, the opposite process to tubular reabsorption. The collecting ducts
collect filtrate coming from the nephrons and this filtrate, called urine, will
be transported into the renal pelvis and then to the ureters. This entire
process is illustrated in [link].
2. Proximal convoluted tubule:
reabsorbs ions, water, and
nutrients; removes toxins
and adjusts filtrate pH
5. Distal tubule:
selectively secretes
and absorbs different
ions to maintain blood
PH and electrolyte
balance
1. Glomerulus:
filters small
solutes from
the blood
6. Collecting
duct:
reabsorbs
solutes and
water from
the filtrate
4. Ascending loop 3. Descending
of Henle: loop of Henle:
reabsorbs Na* and aquaporins
CF from the filtrate allow water
into the interstitial
: to pass from
fluid
) the filtrate
into the
interstitial fluid
Each part of the nephron performs
a different function in filtering
waste and maintaining
homeostatic balance. (1) The
glomerulus forces small solutes
out of the blood by pressure. (2)
The proximal convoluted tubule
reabsorbs ions, water, and
nutrients from the filtrate into the
interstitial fluid, and actively
transports toxins and drugs from
the interstitial fluid into the
filtrate. The proximal convoluted
tubule also adjusts blood pH by
selectively secreting ammonia
(NH3) into the filtrate, where it
reacts with H* to form NH,". The
more acidic the filtrate, the more
ammonia is secreted. (3) The
descending loop of Henle is lined
with cells containing aquaporins
that allow water to pass from the
filtrate into the interstitial fluid.
(4) In the thin part of the
ascending loop of Henle, Na* and
Cl ions diffuse into the interstitial
fluid. In the thick part, these same
ions are actively transported into
the interstitial fluid. Because salt
but not water is lost, the filtrate
becomes more dilute as it travels
up the limb. (5) In the distal
convoluted tubule, K* and H™ ions
are selectively secreted into the
filtrate, while Na‘, Cl’, and HCO3"
ions are reabsorbed to maintain
pH and electrolyte balance in the
blood. (6) The collecting duct
reabsorbs solutes and water from
the filtrate, forming dilute urine.
(credit: modification of work by
NIDDK)
Glomerular Filtration
Glomerular filtration filters out most of the solutes due to high blood
pressure and specialized membranes in the afferent arteriole. The blood
pressure in the glomerulus is maintained independent of factors that affect
systemic blood pressure. The “leaky” connections between the endothelial
cells of the glomerular capillary network allow solutes to pass through
easily. All solutes in the glomerular capillaries, except for macromolecules
like proteins, pass through by passive diffusion. There is no energy
requirement at this stage of the filtration process. Glomerular filtration
rate (GFR) is the volume of glomerular filtrate formed per minute by the
kidneys. GFR is regulated by multiple mechanisms and is an important
indicator of kidney function.
Tubular Reabsorption and Secretion
Tubular reabsorption occurs in the PCT part of the renal tubule. Almost all
nutrients are reabsorbed, and this occurs either by passive or active
transport. Reabsorption of water and some key electrolytes are regulated
and can be influenced by hormones. Sodium (Na’) is the most abundant ion
and most of it is reabsorbed by active transport and then transported to the
peritubular capillaries. Because Na’ is actively transported out of the
tubule, water follows it to even out the osmotic pressure. Water is also
independently reabsorbed into the peritubular capillaries due to the presence
of aquaporins, or water channels, in the PCT.
In the loop of Henle, the permeability of the membrane changes. The
descending limb is permeable to water, not solutes; the opposite is true for
the ascending limb.
Note:
Art Connection
Filtrate enters the Filtrate exits the
descending limb. ascending limb.
Interstitial
fluid
Loop of
Henle
The loop of Henle acts as a
countercurrent multiplier that
uses energy to create
concentration gradients. The
descending limb is water
permeable. Water flows from
the filtrate to the interstitial
fluid, so the concentration of
solutes inside the limb
increases as it descends into
the renal medulla. At the
bottom, the concentration of
solutes is higher inside the
loop than in the interstitial
fluid. Thus, as filtrate enters
the ascending limb, Na* and
Cl ions exit through ion
channels present in the cell
membrane. Further up, Na” is
actively transported out of the
filtrate and Cl follows. The
concentration of solutes is
given in units of milliosmoles
per liter.
By the time the filtrate reaches the DCT, most of the water and solutes have
been reabsorbed. If the body requires additional water, more of it can be
reabsorbed at this point. Further reabsorption is controlled by hormones,
which will be discussed in a later section. Excretion of wastes occurs due to
lack of reabsorption combined with tubular secretion. Undesirable products
like metabolic wastes, urea, uric acid, and certain drugs, are excreted by
tubular secretion. Most of the tubular secretion happens in the DCT, but
some occurs in the early part of the collecting duct. Kidneys also maintain
an acid-base balance by secreting excess H" ions.
Note:
Career Connection
Nephrologist
A nephrologist studies and deals with diseases of the kidneys—both those
that cause kidney failure (such as diabetes) and the conditions that are
produced by kidney disease (such as hypertension). Blood pressure, blood
volume, and changes in electrolyte balance come under the purview of a
nephrologist.
Nephrologists usually work with other physicians who refer patients to
them or consult with them about specific diagnoses and treatment plans.
Patients are usually referred to a nephrologist for symptoms such as blood
or protein in the urine, very high blood pressure, kidney stones, or renal
failure.
Nephrology is a subspecialty of internal medicine. To become a
nephrologist, medical school is followed by additional training to become
certified in internal medicine. An additional two or more years is spent
specifically studying kidney disorders and their accompanying effects on
the body.
Section Summary
The kidneys are the main osmoregulatory organs in mammalian systems;
they function to filter blood and maintain the correct concentration of
solutes in body fluids. They are made up internally of three distinct regions
—the cortex, medulla, and pelvis.
The blood vessels that transport blood into and out of the kidneys arise from
and merge with the aorta and inferior vena cava, respectively. The renal
arteries branch out from the aorta and enter the kidney where they further
divide.
The nephron is the functional unit of the kidney, which actively filters blood
and generates urine. The nephron is made up of the renal corpuscle and
renal tubule. The nephron filters and exchanges water and solutes with two
sets of blood vessels and the tissue fluid in the kidneys.
There are three steps in the formation of urine: glomerular filtration, which
occurs in the glomerulus; tubular reabsorption, which occurs in the renal
tubules; and tubular secretion, which also occurs in the renal tubules.
Art Connections
Exercise:
Problem:
[link] Which of the following statements about the kidney is false?
a. The renal pelvis drains into the ureter.
b. The renal pyramids are in the medulla.
c. The cortex covers the kidney.
d. Nephrons are in the renal cortex.
Solution:
[link] C
Exercise:
Problem:
[link] Which of the following statements about the nephron is false?
a. The collecting duct empties into the distal convoluted tubule.
b. The Bowman’s capsule surrounds the glomerulus.
c. The loop of Henle is between the proximal and distal convoluted
tubules.
d. The loop of Henle empties into the distal convoluted tubule.
Solution:
[link] A
Exercise:
Problem:
[link] Loop diuretics are drugs sometimes used to treat hypertension.
These drugs inhibit the reabsorption of Na* and CI ions by the
ascending limb of the loop of Henle. A side effect is that they increase
urination. Why do you think this is the case?
Solution:
[link] Loop diuretics decrease the excretion of salt into the renal
medulla, thereby reducing its concentration of solutes. As a result, less
water is excreted into the medulla by the descending limb, and more
water is excreted as urine.
Review Questions
Exercise:
Problem:
The gland located at the top of the kidney is the gland.
a. adrenal
b. pituitary
c. thyroid
d. thymus
Solution:
A
Free Response
Exercise:
Problem:
Describe the three major regions of the kidney's internal structure.
Solution:
Internally, the kidney has three regions—an outer cortex, a medulla in
the middle, and the renal pelvis in the region called the hilum of the
kidney, which is the concave part of the “bean” shape.
Glossary
afferent arteriole
arteriole that branches from the cortical radiate artery and enters the
glomerulus
arcuate artery
artery that branches from the interlobar artery and arches over the base
of the renal pyramids
ascending limb
part of the loop of Henle that ascends from the renal medulla to the
renal cortex
Bowman's capsule
structure that encloses the glomerulus
calyx
structure that connects the renal pelvis to the renal medulla
cortex (animal)
outer layer of an organ like the kidney or adrenal gland
cortical nephron
nephron that lies in the renal cortex
cortical radiate artery
artery that radiates from the arcuate arteries into the renal cortex
countercurrent exchanger
peritubular capillary network that allows exchange of solutes and
water from the renal tubules
countercurrent multiplier
osmotic gradient in the renal medulla that is responsible for
concentration of urine
descending limb
part of the loop of Henle that descends from the renal cortex into the
renal medulla
distal convoluted tubule (DCT)
part of the renal tubule that is the most distant from the glomerulus
efferent arteriole
arteriole that exits from the glomerulus
glomerular filtration
filtration of blood in the glomerular capillary network into the
glomerulus
glomerular filtration rate (GFR)
amount of filtrate formed by the glomerulus per minute
glomerulus (renal)
part of the renal corpuscle that contains the capillary network
hilum
region in the renal pelvis where blood vessels, nerves, and ureters
bunch before entering or exiting the kidney
inferior vena cava
one of the main veins in the human body
interlobar artery
artery that branches from the segmental artery and travels in between
the renal lobes
juxtaglomerular cell
cell in the afferent and efferent arterioles that responds to stimuli from
the macula densa
juxtamedullary nephron
nephron that lies in the cortex but close to the renal medulla
kidney
organ that performs excretory and osmoregulatory functions
lobes of the kidney
renal pyramid along with the adjoining cortical region
loop of Henle
part of the renal tubule that loops into the renal medulla
macula densa
group of cells that senses changes in sodium ion concentration; present
in parts of the renal tubule and collecting ducts
medulla
middle layer of an organ like the kidney or adrenal gland
nephron
functional unit of the kidney
perirenal fat capsule
fat layer that suspends the kidneys
peritubular capillary network
capillary network that surrounds the renal tubule after the efferent
artery exits the glomerulus
proximal convoluted tubule (PCT)
part of the renal tubule that lies close to the glomerulus
renal artery
branch of the artery that enters the kidney
renal capsule
layer that encapsulates the kidneys
renal column
area of the kidney through which the interlobar arteries travel in the
process of supplying blood to the renal lobes
renal corpuscle
glomerulus and the Bowman's capsule together
renal fascia
connective tissue that supports the kidneys
renal pelvis
region in the kidney where the calyces join the ureters
renal pyramid
conical structure in the renal medulla
renal tubule
tubule of the nephron that arises from the glomerulus
renal vein
branch of a vein that exits the kidney and joins the inferior vena cava
segmental artery
artery that branches from the renal artery
transport maximum
maximum amount of solute that can be transported out of the renal
tubules during reabsorption
tubular reabsorption
reclamation of water and solutes that got filtered out in the glomerulus
tubular secretion
process of secretion of wastes that do not get reabsorbed
ureter
urine-bearing tube coming out of the kidney; carries urine to the
bladder
urinary bladder
structure that the ureters empty the urine into; stores urine
urine
filtrate produced by kidneys that gets excreted out of the body
vasa recta
peritubular network that surrounds the loop of Henle of the
juxtamedullary nephrons
Hormonal Control of Urine Concentration
By the end of this section, you will be able to:
e Describe how aldosterone and anti-diuretic hormone help control urine
concentration
While the kidneys operate to maintain osmotic balance and blood pressure
in the body, they also act in concert with hormones. Hormones are small
molecules that act as messengers within the body. Hormones are typically
secreted from one cell and travel in the bloodstream to affect a target cell in
another portion of the body. Different regions of the nephron bear
specialized cells that have receptors to respond to chemical messengers and
hormones. In this section, you will learn about two hormones, aldosterone
and antidiuretic hormone, that control urine concentration.
Aldosterone
Aldosterone is a hormone synthesized by the adrenal cortex that affects
urine concentration by regulating sodium levels in the blood. Almost all of
the sodium in the blood is reclaimed by the renal tubules under the
influence of aldosterone. Because sodium is always reabsorbed by active
transport and water follows sodium to maintain osmotic balance,
aldosterone manages not only sodium levels but also the water levels in
urine. Aldosterone favors the production of a concentrated urine by the
water following the reabsorbed sodium ions. A decrease in the secretion of
aldosterone means that less sodium gets reabsorbed in the renal tubules;
therefore, more of it gets excreted in the urine. Patients who have Addison's
disease have a failing adrenal cortex and cannot produce aldosterone. They
lose sodium in their urine constantly, and if the supply is not replenished,
the consequences can be fatal.
Antidiurectic Hormone
Diuretics are drugs that can increase water loss by interfering with the
recapture of solutes and water from the forming urine. They are often
prescribed to lower blood pressure. Coffee, tea, and alcoholic beverages are
familiar diuretics. Antidiuretic hormone or ADH, as the name suggests,
helps the body conserve water when body fluid volume, especially that of
blood, is low. It is formed by the hypothalamus and is stored and released
from the posterior pituitary gland. It acts by inserting aquaporins, protein
channels that allow water to leave, in the collecting ducts and promotes
reabsorption of water. This action results in the formation of a concentrated
urine. ADH also acts as a vasoconstrictor and increases blood pressure
during hemorrhaging.
Section Summary
Hormonal cues help the kidneys synchronize the osmotic needs of the body.
Hormones like aldosterone and anti-diuretic hormone (ADH) help regulate
the needs of the body as well as the communication between the different
organ systems.
Review Questions
Exercise:
Problem: Aldosterone is made by
a. the adrenal glands
b. the hypothalamus
c. the anterior pituitary gland
d. the posterior pituitary gland
Solution:
A
Exercise:
Problem: Patients with Addison's disease
a. retain water
b. retain salts
c. lose salts and water
d. have too much aldosterone
Solution:
C
Free Response
Exercise:
Problem:
Describe how hormones regulate blood pressure, blood volume, and
kidney function.
Solution:
Hormones are small molecules that act as messengers within the body.
Different regions of the nephron bear specialized cells, which have
receptors to respond to chemical messengers and hormones. The
hormones carry messages to the kidney. These hormonal cues help the
kidneys synchronize the osmotic needs of the body. Both ADH and
aldosterone promote water reabsorption from the filtrate, which
increases blood volume and blood pressure while also producing a
concentrated urine.
Glossary
angiotensin converting enzyme (ACE)
enzyme that converts angiotensin I to angiotensin II
angiotensin I
product in the renin-angiotensin-aldosterone pathway
angiotensin II
molecule that affects different organs to increase blood pressure
anti-diuretic hormone (ADH)
hormone that prevents the loss of water
renin-angiotensin-aldosterone
biochemical pathway that activates angiotensin II, which increases
blood pressure
vasodilator
compound that increases the diameter of blood vessels
vasopressin
another name for anti-diuretic hormone
Introduction to Cell Division
class="introduction"
A sea urchin
begins life
as a single
cell that (a)
divides to
form two
cells, visible
by scanning
electron
microscopy.
After four
rounds of
cell
division, (b)
there are 16
cells, as
seen in this
SEM image.
After many
rounds of
cell
division, the
individual
develops
into a
complex,
multicellula
r organism,
as seen in
this (c)
mature sea
urchin.
(credit a:
modificatio
n of work
by Evelyn
Spiegel,
Louisa
Howard;
credit b:
modificatio
n of work
by Evelyn
Spiegel,
Louisa
Howard;
credit c:
modificatio
n of work
by Marco
Busdraghi;
scale-bar
data from
Matt
Russell)
(b)
The individual sexually reproducing organism—including humans—begins
life as a fertilized egg, or zygote. Trillions of cell divisions subsequently
occur in a controlled manner to produce a complex, multicellular human. In
other words, that original single cell was the ancestor of every other cell in
the body. Once a human individual is fully grown, cell reproduction is still
necessary to repair or regenerate tissues. For example, new blood and skin
cells are constantly being produced. The type of cell division associated
with these events is mitosis, which produces genetically-identical cells with
two sets of chromosomes (i.e. diploid). However, Humans also have to be
able to produce specialized cells for reproduction (i.e. gametes) that contain
only one set of chromosomes (i.e. haploid). The type of cell division
associated with gamete production is meiosis.
Chromosomes and the Genome
By the end of this section, you will be able to:
e Describe the eukaryotic genome
e Distinguish between chromosomes, genes, and traits
The continuity of life from one cell to another has its foundation in the
reproduction of cells by way of the cell cycle. The cell cycle is an orderly
sequence of events in the life of a cell from the division of a single parent
cell to produce two new daughter cells, to the subsequent division of those
daughter cells. The mechanisms involved in the cell cycle are highly
conserved across eukaryotes.
Genomic DNA
Before discussing the steps a cell undertakes to replicate, a deeper
understanding of the structure and function of a cell’s genetic information is
necessary. A complement of DNA in a gamete is referred to as the genome.
A somatic cell (i.e. a cell with two sets of chromosomes) contains 2 copies
of the genome - one from the mother's egg and one from the father's sperm.
These two copies of the genome are found in the zygote.
In eukaryotes, the genome comprises several double-stranded, linear DNA
molecules ({link]) bound with proteins to form complexes called
chromosomes. Each species of eukaryote has a characteristic number of
chromosomes in the nuclei of its cells. Human body cells (somatic cells)
have 46 chromosomes. A somatic cell contains two matched sets of
chromosomes, a configuration known as diploid. The letter n is used to
represent a single set of chromosomes; therefore a diploid organism is
designated 2n. Human cells that contain one set of 23 chromosomes are
called gametes, or sex cells; these eggs and sperm are designated n, or
haploid.
‘6
6 7 8 9 10 11 12
+f ih
13 14 15 16 17 18
19 20 21 22 X
There are 23 pairs of homologous
chromosomes in a female human
somatic cell. These chromosomes
are viewed within the nucleus
(top), removed from a cell in
mitosis (right), and arranged
according to length (left) in an
arrangement called a karyotype. In
the karyotype, the first 22 pairs of
chromosomes are called
autosomes. The 23rd pair of
chromosomes is called the sex
chromosomes. A male has the sex
chromosomes X and Y and the
female has the sex chromosomes
X and X. Therefore, the human
karyotype shown is from a female.
In this image, the chromosomes
were exposed to fluorescent stains
to distinguish them. (credit: “718
Bot”/Wikimedia Commons,
National Human Genome
Research)
The matched pairs of chromosomes in a diploid organism are called
homologous chromosomes. Homologous chromosomes are the same
length and have specific nucleotide segments called genes in exactly the
same location, or loci; singular: locus. Genes, the functional units of
chromosomes, determine specific characteristics by coding for specific
proteins. Traits are the different forms of a characteristic. For example, the
shape of earlobes is a characteristic with traits of free or attached.
Each copy of the homologous pair of chromosomes originates from a
different parent; therefore, the copies of each of the genes themselves may
not be identical. The variation of individuals within a species is caused by
the specific combination of the genes inherited from both parents. For
example, there are three possible gene sequences on the human
chromosome that codes for blood type: sequence A, sequence B, and
sequence O. Because all diploid human cells have two copies of the
chromosome that determines blood type, the blood type (the trait) is
determined by which two versions of the marker gene are inherited. It is
possible to have two copies of the same gene sequence, one on each
homologous chromosome (for example, AA, BB, or OO), or two different
sequences, such as AB.
Minor variations in traits such as those for blood type, eye color, and height
contribute to the natural variation found within a species. The sex
chromosomes, X and Y, are the single exception to the rule of homologous
chromosomes; other than a small amount of homology that is necessary to
reliably produce gametes, the genes found on the X and Y chromosomes are
not the same.
Section Summary
Eukaryotes have multiple, linear chromosomes surrounded by a nuclear
membrane. Human somatic cells have 46 chromosomes consisting of two
sets of 22 homologous chromosomes and a pair of nonhomologous sex
chromosomes. This is the 2n, or diploid, state. Human gametes have 23
chromosomes or one complete set of chromosomes. This is the n, or
haploid, state. Genes are segments of DNA that code for a specific protein
or RNA molecule. An organism’s traits are determined in large part by the
genes inherited from each parent, but also by the environment that they
experience. Genes are expressed as characteristics of the organism and each
characteristic may have different variants called traits that are caused by
differences in the DNA sequence for a gene.
Multiple Choice
Exercise:
Problem:
A diploid cell has the number of chromosomes as a haploid
cell.
a. one-fourth
b. one-half
c. twice
d. four times
Solution:
CG
Exercise:
Problem:
An organism’s traits are determined by the specific combination of
inherited
a. cells
b. genes
c. proteins
d. chromatids
Solution:
Free Response
Exercise:
Problem:
Compare and contrast a human somatic cell to a human gamete.
Solution:
Human somatic cells have 46 chromosomes, including 22 homologous
pairs and one pair of nonhomologous sex chromosomes. This is the 2n,
or diploid, condition. Human gametes have 23 chromosomes, one each
of 23 unique chromosomes. This is the n, or haploid, condition.
Glossary
diploid
describes a cell, nucleus, or organism containing two sets of
chromosomes (2n)
gamete
a haploid reproductive cell or sex cell (sperm or egg)
gene
the physical and functional unit of heredity; a sequence of DNA that
codes for a specific peptide or RNA molecule
genome
the entire genetic complement (DNA) of an organism
haploid
describes a cell, nucleus, or organism containing one set of
chromosomes (n)
homologous chromosomes
chromosomes of the same length with genes in the same location;
diploid organisms have pairs of homologous chromosomes, and the
members of each pair come from different parents
locus
the position of a gene on a chromosome
The Cell Cycle
By the end of this section, you will be able to:
e Describe the three stages of interphase
e Discuss the behavior of chromosomes during mitosis and how the
cytoplasmic content divides during cytokinesis
e Define the quiescent Gp phase
e Explain how the three internal control checkpoints occur at the end of
Gj, at the G>—M transition, and during metaphase
The cell cycle is an ordered series of events involving cell growth and cell
division (i.e. Mitosis) that produces two new daughter cells. Cells on the
path to cell division proceed through a series of precisely timed and
carefully regulated stages of growth, DNA replication, and division that
produce two genetically identical cells. The cell cycle has two major
phases: interphase and the mitotic phase ({link]). During (interphase, the
cell grows and DNA is replicated. During the mitotic phase, the replicated
chromosomes separate (via mitosis), the cytoplasm divides (via cytokinesis,
and the cell formally divides into two daughter cells. Watch this video about
the cell cycle: https://www. youtube.com/watch?v=Wy3N5NCZBHQ
Mitotic Phase
Interphase
Mitosis Formation
Cytokinesis of 2 daughter
cells
Interphase Interphase
A cell moves through a series of phases in an orderly
manner. During interphase, G involves cell growth and
protein synthesis, the S phase involves DNA replication
and the replication of the centrosome, and G> involves
further growth and protein synthesis. The mitotic phase
follows interphase. Mitosis is nuclear division during
which duplicated chromosomes are segregated and
distributed into daughter nuclei. Usually the cell will
divide after mitosis in a process called cytokinesis in
which the cytoplasm is divided and two daughter cells
are formed.
Interphase
During interphase, the cell undergoes normal processes while also preparing
for cell division. For a cell to move from interphase to the mitotic phase,
many internal and external conditions must be met. The three stages of
interphase are called G, S, and Gp.
G, Phase
The first stage of interphase is called the G, phase, or first gap, because
little change is visible. However, during the G, stage, the cell is quite active
at the biochemical level. The cell is accumulating the building blocks of
chromosomal DNA and the associated proteins, as well as accumulating
enough energy reserves to complete the task of replicating each
chromosome in the nucleus.
S Phase
Throughout interphase, nuclear DNA remains in a semi-condensed
chromatin configuration. In the S phase (synthesis phase), DNA replication
results in the formation of two identical copies of each chromosome—-sister
chromatids—that are firmly attached at the centromere region. At this stage,
each chromosome is made of two sister chromatids and is a duplicated
chromosome. The centrosome is duplicated during the S phase.
G> Phase
In the Gy phase, or second gap, the cell replenishes its energy stores and
synthesizes the proteins necessary for chromosome manipulation. Some cell
organelles are duplicated, and the cytoskeleton is dismantled to provide
resources for the mitotic spindle. There may be additional cell growth
during Gp. The final preparations for the mitotic phase must be completed
before the cell is able to enter the first stage of mitosis.
The Mitotic Phase
To make two daughter cells, the contents of the nucleus and the cytoplasm
must be divided. The mitotic phase is a multistep process during which the
duplicated chromosomes are aligned, separated, and moved to opposite
poles of the cell, and then the cell is divided into two new identical daughter
cells. The first portion of the mitotic phase, mitosis, is composed of five
stages, which accomplish nuclear division. The second portion of the
mitotic phase, called cytokinesis, is the physical separation of the
cytoplasmic components into two daughter cells.
Mitosis
Mitosis is divided into a series of phases—prophase, prometaphase,
metaphase, anaphase, and telophase—that result in the division of the cell
nucleus ((Llink]).
Note:
Art Connection
+ Animal cells: a
cleavage furrow
separates the
daughter cells
* Chromosomes
arrive at opposite
poles and begin
to decondense
* Chromosomes
continue to
condense
* Chromosomes
condense and
become visible
Mitotic spindle is | * Cohesin proteins
fully developed, binding the sister
centrosomes are} chromatids
at opposite poles} together break
* Kinetochores of the cell down
appear at the
centromeres
* Spindle fibers
emerge from the
centrosomes
* Nuclear envelope | « Plant cells: a cell
Chromosomes * Sister chromatids} material
* Nuclear envelope
breaks down
* Nucleolus
disappears
* Mitotic spindle
microtubules
attach to
kinetochores
* Centrosomes
move toward
opposite poles
are lined up at
the metaphase
plate
Each sister
chromatid is
attached toa
spindle fiber
originating from
(now called
chromosomes)
are pulled toward
opposite poles
+ Non-kinetochore
spindle fibers
lengthen,
elongating
the cell
surrounds
each set of
chromosomes
* The mitotic
spindle breaks
down
plate separates
the daughter
cells
opposite poles
MITOSIS
Animal cell mitosis is divided into five stages—prophase,
prometaphase, metaphase, anaphase, and telophase—visualized
here by light microscopy with fluorescence. Mitosis is usually
accompanied by cytokinesis, shown here by a transmission
electron microscope. (credit "diagrams": modification of work
by Mariana Ruiz Villareal; credit "mitosis micrographs":
modification of work by Roy van Heesbeen; credit "cytokinesis
micrograph": modification of work by the Wadsworth Center,
NY State Department of Health; donated to the Wikimedia
foundation; scale-bar data from Matt Russell)
During prophase, the “first phase,” several events must occur to provide
access to the chromosomes in the nucleus. The nuclear envelope starts to
break into small vesicles, and the Golgi apparatus and endoplasmic
reticulum fragment and disperse to the periphery of the cell. The nucleolus
disappears. The centrosomes begin to move to opposite poles of the cell.
The microtubules that form the basis of the mitotic spindle extend between
the centrosomes, pushing them farther apart as the microtubule fibers
lengthen. The sister chromatids begin to coil more tightly and become
visible under a light microscope.
During prometaphase, many processes that were begun in prophase
continue to advance and culminate in the formation of a connection
between the chromosomes and cytoskeleton. The remnants of the nuclear
envelope disappear. The mitotic spindle continues to develop as more
microtubules assemble and stretch across the length of the former nuclear
area. Chromosomes become more condensed and visually discrete. Each
sister chromatid attaches to spindle microtubules at the centromere via a
protein complex called the kinetochore.
During metaphase, all of the chromosomes are aligned in a plane called the
metaphase plate, or the equatorial plane, midway between the two poles of
the cell. The sister chromatids are still tightly attached to each other. At this
time, the chromosomes are maximally condensed.
During anaphase, the sister chromatids at the equatorial plane are split
apart at the centromere. Each chromatid, now called a chromosome, is
pulled rapidly toward the centrosome to which its microtubule was
attached. The cell becomes visibly elongated as the non-kinetochore
microtubules slide against each other at the metaphase plate where they
overlap.
During telophase, all of the events that set up the duplicated chromosomes
for mitosis during the first three phases are reversed. The chromosomes
reach the opposite poles and begin to decondense (unravel). The mitotic
spindles are broken down into monomers that will be used to assemble
cytoskeleton components for each daughter cell. Nuclear envelopes form
around chromosomes.
Cytokinesis
Cytokinesis is the second part of the mitotic phase during which cell
division is completed by the physical separation of the cytoplasmic
components into two daughter cells. Although the stages of mitosis are
similar for most eukaryotes, the process of cytokinesis is quite different for
eukaryotes that have cell walls, such as plant cells.
In cells such as animal cells that lack cell walls, cytokinesis begins
following the onset of anaphase. A contractile ring composed of actin
filaments forms just inside the plasma membrane at the former metaphase
plate. The actin filaments pull the equator of the cell inward, forming a
fissure. This fissure, or “crack,” is called the cleavage furrow. The furrow
deepens as the actin ring contracts, and eventually the membrane and cell
are cleaved in two ([link]).
(a) Animal cell
Cleavage
furrow E>
Contractile
ring
Cell
2 ~
Golgi vesicles
(b) Plant cell
In part (a), a cleavage furrow forms at
the former metaphase plate in the
animal cell. The plasma membrane is
drawn in by a ring of fibers
contracting just inside the membrane.
The cleavage furrow deepens until the
cells are pinched in two.
Section Summary
The cell cycle is an orderly sequence of events. Cells on the path to cell
division proceed through a series of precisely timed and carefully regulated
stages. In eukaryotes, the cell cycle consists of a long preparatory period,
called interphase. Interphase is divided into Gj, S, and G» phases. Mitosis
consists of five stages: prophase, prometaphase, metaphase, anaphase, and
telophase. Mitosis is usually accompanied by cytokinesis, during which the
cytoplasmic components of the daughter cells are separated either by an
actin ring (animal cells) or by cell plate formation (plant cells).
Art Connections
Exercise:
Problem:
[link] Which of the following is the correct order of events in mitosis?
a. Sister chromatids line up at the metaphase plate. The kinetochore
becomes attached to the mitotic spindle. The nucleus re-forms
and the cell divides. The sister chromatids separate.
b. The kinetochore becomes attached to the mitotic spindle. The
sister chromatids separate. Sister chromatids line up at the
metaphase plate. The nucleus re-forms and the cell divides.
c. The kinetochore becomes attached to metaphase plate. Sister
chromatids line up at the metaphase plate. The kinetochore breaks
down and the sister chromatids separate. The nucleus re-forms
and the cell divides.
d. The kinetochore becomes attached to the mitotic spindle. Sister
chromatids line up at the metaphase plate. The kinetochore breaks
apart and the sister chromatids separate. The nucleus re-forms and
the cell divides.
Solution:
[link] D. The kinetochore becomes attached to the mitotic spindle.
Sister chromatids line up at the metaphase plate. The kinetochore
breaks apart and the sister chromatids separate. The nucleus reforms
and the cell divides.
Multiple Choice
Exercise:
Problem:
Chromosomes are duplicated during what portion of the cell cycle?
a. G, phase
b. S phase
c. prophase
d. prometaphase
Solution:
B
Exercise:
Problem:
Separation of the sister chromatids is a characteristic of which stage of
mitosis?
a. prometaphase
b. metaphase
c. anaphase
d. telophase
Solution:
C
Exercise:
Problem:
The individual chromosomes become visible with a light microscope
during which stage of mitosis?
a. prophase
b. prometaphase
c. metaphase
d. anaphase
Solution:
A
Glossary
anaphase
the stage of mitosis during which sister chromatids are separated from
each other
cell cycle
the ordered sequence of events that a cell passes through between one
cell division and the next
cell cycle checkpoints
mechanisms that monitor the preparedness of a eukaryotic cell to
advance through the various cell cycle stages
cell plate
a structure formed during plant-cell cytokinesis by Golgi vesicles
fusing at the metaphase plate; will ultimately lead to formation of a
cell wall to separate the two daughter cells
centriole
a paired rod-like structure constructed of microtubules at the center of
each animal cell centrosome
cleavage furrow
a constriction formed by the actin ring during animal-cell cytokinesis
that leads to cytoplasmic division
cytokinesis
the division of the cytoplasm following mitosis to form two daughter
cells
Go phase
a cell-cycle phase distinct from the G, phase of interphase; a cell in Gg
is not preparing to divide
G, phase
(also, first gap) a cell-cycle phase; first phase of interphase centered on
cell growth during mitosis
G> phase
(also, second gap) a cell-cycle phase; third phase of interphase where
the cell undergoes the final preparations for mitosis
interphase
the period of the cell cycle leading up to mitosis; includes G,, S, and
G, phases; the interim between two consecutive cell divisions
kinetochore
a protein structure in the centromere of each sister chromatid that
attracts and binds spindle microtubules during prometaphase
metaphase plate
the equatorial plane midway between two poles of a cell where the
chromosomes align during metaphase
metaphase
the stage of mitosis during which chromosomes are lined up at the
metaphase plate
mitosis
the period of the cell cycle at which the duplicated chromosomes are
separated into identical nuclei; includes prophase, prometaphase,
metaphase, anaphase, and telophase
mitotic phase
the period of the cell cycle when duplicated chromosomes are
distributed into two nuclei and the cytoplasmic contents are divided;
includes mitosis and cytokinesis
mitotic spindle
the microtubule apparatus that orchestrates the movement of
chromosomes during mitosis
prometaphase
the stage of mitosis during which mitotic spindle fibers attach to
kinetochores
prophase
the stage of mitosis during which chromosomes condense and the
mitotic spindle begins to form
quiescent
describes a cell that is performing normal cell functions and has not
initiated preparations for cell division
S phase
the second, or synthesis phase, of interphase during which DNA
replication occurs
telophase
the stage of mitosis during which chromosomes arrive at opposite
poles, decondense, and are surrounded by new nuclear envelopes
Meiosis and Genetic Variation
By the end of this section, you will be able to:
¢ Describe the behavior of chromosomes during meiosis
e Describe cellular events during meiosis
e Explain the differences between meiosis and mitosis
e Explain the mechanisms within meiosis that generate genetic variation
among the products of meiosis
Sexual reproduction requires fertilization, a union of two haploid cells (i.e.
gametes) from two individual organisms. If those two cells each contain one
set of chromosomes, then the resulting cell contains two sets of
chromosomes (i.e. is diploid). The number of sets of chromosomes in a cell
is called its ploidy level. Haploid cells contain one set of chromosomes.
Cells containing two sets of chromosomes are called diploid. If the
reproductive cycle is to continue, the diploid cell must somehow reduce its
number of chromosome sets before fertilization can occur again, or there
will be a continual doubling in the number of chromosome sets in every
generation. So, in addition to fertilization, sexual reproduction includes a
nuclear division, known as meiosis, that reduces the number of
chromosome sets. Meiosis consists of two division events, called Meiosis I
and Meiosis II.
Most animals and plants are diploid, containing two sets of chromosomes;
in each somatic cell (the nonreproductive cells of a multicellular organism),
the nucleus contains two copies of each chromosome that are referred to as
homologous chromosomes. Somatic cells are sometimes referred to as
“body” cells. Homologous chromosomes are matched pairs containing
genes for the same traits in identical locations along their length. Diploid
organisms inherit one copy of each homologous chromosome from each
parent; all together, they are considered a full set of chromosomes. In
animals, haploid cells containing a single copy of each homologous
chromosome are found only within gametes. Gametes fuse with another
haploid gamete to produce a diploid cell.
The nuclear division that forms haploid cells, which is called meiosis, is
related to mitosis. As you have learned, mitosis is part of a cell reproduction
cycle that results in identical daughter nuclei that are also genetically
identical to the original parent nucleus. In mitosis, both the parent and the
daughter nuclei contain the same number of chromosome sets—diploid for
most plants and animals. Meiosis employs many of the same mechanisms
as mitosis. However, the starting nucleus is always diploid and the nuclei
that result at the end of a meiotic cell division are haploid (n). To achieve
the reduction in chromosome number, meiosis consists of one round of
chromosome duplication and two rounds of nuclear division. Because the
events that occur during each of the division stages are analogous to the
events of mitosis, the same stage names are assigned. However, because
there are two rounds of division, the stages are designated with a “I” or “II.”
Thus, meiosis I is the first round of meiotic division and consists of
prophase I, prometaphase I, and so on. Meiosis I reduces the number of
chromosome sets from two to one (i.e. reductional division). The genetic
information is also mixed during this division to create unique recombinant
chromosomes. Meiosis II, in which the second round of meiotic division
takes place in a way that is similar to mitosis, includes prophase II,
prometaphase II, and so on. Meiosis II produces daughter cells that are
haploid in chromosome number (as in Meiosis I), but with the sister
chromatids separated. In other words, there is no further reduction in
chromosome number, thus it is also called equational division.
Interphase
Meiosis is preceded by an interphase consisting of the Gj, S, and Gp phases,
which are nearly identical to the phases preceding mitosis. The G, phase is
the first phase of interphase and is focused on cell growth. In the S phase,
the DNA of the chromosomes is replicated. Finally, in the G» phase, the cell
undergoes the final preparations for meiosis.
During DNA duplication of the S phase, each chromosome becomes
composed of two identical copies (called sister chromatids) that are held
together at the centromere until they are pulled apart during meiosis II.
Again, homologous chromosome pairs separate in meiosis I (i.e. reductional
division) and sister chromatids separate during meiosis II (i.e. equational
division).
Meiosis I
Early in prophase I, the chromosomes can be seen clearly microscopically.
As the nuclear envelope begins to break down, the proteins associated with
homologous chromosomes bring the pair close to each other. The tight
pairing of the homologous chromosomes is called synapsis. In synapsis, the
genes on the chromatids of the homologous chromosomes are precisely
aligned with each other. An exchange of chromosome segments between
non-sister homologous chromatids occurs and is called crossing over. This
process is revealed visually after the exchange as chiasmata (singular =
chiasma) ((link]). As will be discussed later, crossing over can result in
genetic variability in the gametes.
As prophase I progresses, the close association between homologous
chromosomes begins to break down, and the chromosomes continue to
condense, although the homologous chromosomes remain attached to each
other at chiasmata. The number of chiasmata varies with the species and the
length of the chromosome. At the end of prophase I, the pairs are held
together only at chiasmata ((link]) and are called tetrads because the four
sister chromatids of each pair of homologous chromosomes are now visible.
The crossover events are the first source of genetic variation produced by
meiosis. A single crossover event between homologous non-sister
chromatids leads to a reciprocal exchange of equivalent DNA between a
maternal chromosome and a paternal chromosome. Now, when that sister
chromatid is moved into a gamete, it will carry some DNA from one parent
of the individual and some DNA from the other parent. The recombinant
sister chromatid has a combination of maternal and paternal genes that did
not exist before the crossover. It is important to note that crossing over will
only produce genetic diversity if there was diversity between the maternal
and paternal chromosomes.
Homologous Chromosome
chromosomes crossover
aligned
Recombinant
chromosomes
Non-recombinant
chromosomes
In this illustration of the effects
of crossing over, the blue
chromosome came from the
individual’s father and the red
chromosome came from the
individual’s mother. Crossover
occurs between non-sister
chromatids of homologous
chromosomes. The result is an
exchange of genetic material
between homologous
chromosomes. The
chromosomes that have a
mixture of maternal and
paternal sequence that differ
genetically are called
recombinant and the
chromosomes that are
completely paternal or maternal
are called non-recombinant.
Note: Crossing over can occur
several times between the same
pair of homologous
chromosomes.
During metaphase I, the homologous chromosomes are arranged in the
center of the cell with the kinetochores facing opposite poles. The
orientation of each pair of homologous chromosomes at the center of the
cell is random. As is discussed later, this too can contribute to genetic
variation in the gametes.
This randomness, called independent assortment, is the physical basis for
the generation of the second form of genetic variation in offspring. Consider
that the homologous chromosomes of a sexually reproducing organism are
originally inherited as two separate sets, one from each parent. Using
humans as an example, one set of 23 chromosomes is present in the egg
donated by the mother. The father provides the other set of 23 chromosomes
in the sperm that fertilizes the egg. In metaphase I, these pairs line up at the
midway point between the two poles of the cell. Because there is an equal
chance that a microtubule fiber will encounter a maternally or paternally
inherited chromosome, the arrangement of the tetrads at the metaphase plate
is random. Any maternally inherited chromosome may face either pole.
Any paternally inherited chromosome may also face either pole. The
orientation of each tetrad is independent of the orientation of the other 22
tetrads.
In each cell that undergoes meiosis, the arrangement of the tetrads is
different. The number of variations depends on the number of chromosomes
making up a set. There are two possibilities for orientation (for each tetrad);
thus, the possible number of alignments equals 2” where n is the number of
chromosomes per set. Humans have 23 chromosome pairs, which results in
over eight million (27°) possibilities. This number does not include the
variability previously created in the sister chromatids by crossover. Given
these two mechanisms, it is highly unlikely that any two haploid cells
resulting from meiosis will have the same genetic composition ({link]).
To summarize the genetic consequences of meiosis I: the maternal and
paternal genes are recombined by crossover events occurring on each
homologous pair during prophase I; in addition, the random assortment of
tetrads at metaphase produces a unique combination of maternal and
paternal chromosomes that will make their way into the gametes.
Metaphase II
Gametes
Chromosome Arrangement 1
arrangement 2
Metaphase II
Genetic Genetic
arrangement 3 arrangement 4
To demonstrate random, independent assortment at
metaphase I, consider a cell with n = 2. In this
case, there are two possible arrangements at the
equatorial plane in metaphase I, as shown in the
upper cell of each panel. These two possible
orientations lead to the production of genetically
different gametes. With more chromosomes, the
i
i
i
number of possible arrangements increases
dramatically.
In anaphase I, the spindle fibers pull the linked chromosomes apart. The
sister chromatids remain tightly bound together at the centromere. It is the
chiasma connections that are broken in anaphase I as the fibers attached to
the fused kinetochores pull the homologous chromosomes apart ((link]).
In telophase I, the separated chromosomes arrive at opposite poles. The
remainder of the typical telophase events may or may not occur depending
on the species. In some organisms, the chromosomes decondense and
nuclear envelopes form around the chromatids in telophase I.
Cytokinesis, the physical separation of the cytoplasmic components into
two daughter cells, occurs without reformation of the nuclei in other
organisms. In nearly all animals, cytokinesis separates the cell contents by a
cleavage furrow. At each pole, there is just one member of each pair of the
homologous chromosomes, so only one full set of the chromosomes is
present. This is why the cells are considered haploid—there is only one
chromosome set, even though there are duplicate copies of the set because
each homolog still consists of two sister chromatids that are still attached to
each other. However, although the sister chromatids were once duplicates of
the same chromosome, they are no longer identical at this stage because of
crossovers.
Note:
Concept in Action
ee
openstax COLLEGE
Review the process of meiosis, observing how chromosomes align and
migrate, at this site.
Meiosis II
In meiosis II, the connected sister chromatids remaining in the haploid cells
from meiosis I will be split to form four haploid cells. In some species, cells
enter a brief interphase, or interkinesis, that lacks an S phase, before
entering meiosis II. Chromosomes are not duplicated during interkinesis.
The two cells produced in meiosis I go through the events of meiosis IT in
synchrony. Overall, meiosis II resembles the mitotic division of a haploid
cell.
In prophase II, if the chromosomes decondensed in telophase I, they
condense again. If nuclear envelopes were formed, they fragment into
vesicles. The centrosomes duplicated during interkinesis move away from
each other toward opposite poles, and new spindles are formed. In
prometaphase II, the nuclear envelopes are completely broken down, and
the spindle is fully formed. Each sister chromatid forms an individual
kinetochore that attaches to microtubules from opposite poles. In metaphase
II, the sister chromatids are maximally condensed and aligned at the center
of the cell. In anaphase II, the sister chromatids are pulled apart by the
spindle fibers and move toward opposite poles.
Prometaphase | Anaphase |
a | Homologous pairs of
~ Homologous — chromosomes are pulled
_ pairs of apart by microtubules
chromosomes { attached to the kinetochore.
| are held
together at the
chiasmata.
MEIOSIS |
View nines attach | Sister chromatids |
to the fused kinetochores remain attached at
of the sister chromatids. _ the centromere.
Prometaphase II Anaphase II
‘Sister chromatids are
pulled apart by microtubules
| attached to — kinetochore.
"Sister chromatids
are held together
_ at the centromere.
Microtubules attach to the
individual kinetochores of
the sister chromatids.
In prometaphase I, microtubules attach to the fused
kinetochores of homologous chromosomes. In
anaphase I, the homologous chromosomes are
separated. In prometaphase II, microtubules attach
to individual kinetochores of sister chromatids. In
anaphase II, the sister chromatids are separated.
In telophase II, the chromosomes arrive at opposite poles and begin to
decondense. Nuclear envelopes form around the chromosomes. Cytokinesis
separates the two cells into four genetically unique haploid cells. At this
point, the nuclei in the newly produced cells are both haploid and have only
one copy of the single set of chromosomes. The cells produced are
genetically unique because (assuming there was parental genetic variation)
of the random assortment of paternal and maternal homologs and because
of the recombination of maternal and paternal segments of chromosomes—
with their sets of genes—that occurs during crossover.
Comparing Meiosis and Mitosis
Mitosis and meiosis, which are both forms of division of the nucleus in
eukaryotic cells, share some similarities, but also exhibit distinct differences
that lead to their very different outcomes. Mitosis is a single nuclear
division that results in two nuclei, usually partitioned into two new cells.
The nuclei resulting from a mitotic division are genetically identical to the
original. They have the same number of sets of chromosomes: one in the
case of haploid cells, and two in the case of diploid cells. On the other hand,
meiosis is two nuclear divisions that result in four nuclei, usually
partitioned into four new cells. The nuclei resulting from meiosis are never
genetically identical, and they contain one chromosome set only—this is
half the number of the original cell, which was diploid ((link]).
The differences in the outcomes of meiosis and mitosis occur because of
differences in the behavior of the chromosomes during each process. Most
of these differences in the processes occur in meiosis I, which is a very
different nuclear division than mitosis. In meiosis I, the homologous
chromosome pairs become associated with each other, are bound together,
experience chiasmata and crossover between sister chromatids, and line up
along the metaphase plate in tetrads with spindle fibers from opposite
spindle poles attached to each kinetochore of a homolog in a tetrad. All of
these events occur only in meiosis I, never in mitosis.
Homologous chromosomes move to opposite poles during meiosis I so the
number of sets of chromosomes in each nucleus-to-be is reduced from two
to one. For this reason, meiosis I is referred to as a reduction division.
There is no such reduction in ploidy level in mitosis.
Meiosis II is much more analogous to a mitotic division. In this case,
duplicated chromosomes (only one set of them) line up at the center of the
cell with divided kinetochores attached to spindle fibers from opposite
poles. During anaphase II, as in mitotic anaphase, the kinetochores divide
and one sister chromatid is pulled to one pole and the other sister chromatid
is pulled to the other pole. If it were not for the fact that there had been
crossovers, the two products of each meiosis IT division would be identical
as in mitosis; instead, they are different because there has always been at
least one crossover per chromosome. Meiosis II is not a reduction division
because, although there are fewer copies of the genome in the resulting
cells, there is still one set of chromosomes, as there was at the end of
meiosis I.
Cells produced by mitosis will function in different parts of the body as a
part of growth or replacing dead or damaged cells. Cells produced by
meiosis in animals will only participate in sexual reproduction.
HAPLOID CELLS
Meiosis | Meiosis II Cytokinesis
Prometaphase | Anaphase |
Prophase | Metaphase | Telophase | G R
( is /
Cytokinesis @
DIPLOID CELLS
DNA Synapsis of Crossover Homologous Sister chromatids Number
synthesis homologous chromosomes line up at and genetic
chromosomes line up at metaphase plate composition of
metaphase plate daughter cells
Occurs in S phase During During During During Four haploid
of interphase prophase | prophase | metaphase | metaphase II cells at the end
of meiosis II
Occurs in S phase Does not Does not Does not During Two diploid
of interphase occur occur occur metaphase cells at the end
in mitosis in mitosis in mitosis of mitosis
Meiosis and mitosis are both preceded by one
round of DNA replication; however, meiosis
includes two nuclear divisions. The four daughter
cells resulting from meiosis are haploid and
genetically distinct. The daughter cells resulting
from mitosis are diploid and identical to the parent
cell.
Note:
Concept in Action
oO)
openstax COLLEGE”
:
a7
a
For an animation comparing mitosis and meiosis, go to this website.
Section Summary
Sexual reproduction requires that diploid organisms produce haploid cells
that can fuse during fertilization to form diploid offspring. The process that
results in haploid cells is called meiosis. Meiosis is a series of events that
arrange and separate chromosomes into daughter cells. During the
interphase of meiosis, each chromosome is duplicated. In meiosis, there are
two rounds of nuclear division resulting in four nuclei and usually four
haploid daughter cells, each with half the number of chromosomes as the
parent cell. During meiosis, variation in the daughter nuclei is introduced
because of crossover in prophase I and random alignment at metaphase I.
The cells that are produced by meiosis are genetically unique.
Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic
divisions are single nuclear divisions that produce daughter nuclei that are
genetically identical and have the same number of chromosome sets as the
original cell. Meiotic divisions are two nuclear divisions that produce four
daughter nuclei that are genetically different and have one chromosome set
rather than the two sets the parent cell had. The main differences between
the processes occur in the first division of meiosis. The homologous
chromosomes separate into different nuclei during meiosis I causing a
reduction of ploidy level. The second division of meiosis is much more
similar to a mitotic division.
Multiple Choice
Exercise:
Problem: Meiosis produces daughter cells.
a. two haploid
b. two diploid
c. four haploid
d. four diploid
Solution:
C
Exercise:
Problem:
At which stage of meiosis are sister chromatids separated from each
other?
a. prophase I
b. prophase II
c. anaphase I
d. anaphase II
Solution:
D
Exercise:
Problem: The part of meiosis that is similar to mitosis is
a. meiosis |
b. anaphase I
c. meiosis II
d. interkinesis
Solution:
C
Exercise:
Problem:
If a muscle cell of a typical organism has 32 chromosomes, how many
chromosomes will be in a gamete of that same organism?
Solution:
B
Free Response
Exercise:
Problem:
Explain how the random alignment of homologous chromosomes
during metaphase I contributes to variation in gametes produced by
meiosis.
Solution:
Random alignment leads to new combinations of traits. The
chromosomes that were originally inherited by the gamete-producing
individual came equally from the egg and the sperm. In metaphase I,
the duplicated copies of these maternal and paternal homologous
chromosomes line up across the center of the cell to form a tetrad. The
orientation of each tetrad is random. There is an equal chance that the
maternally derived chromosomes will be facing either pole. The same
is true of the paternally derived chromosomes. The alignment should
occur differently in almost every meiosis. As the homologous
chromosomes are pulled apart in anaphase I, any combination of
maternal and paternal chromosomes will move toward each pole. The
gametes formed from these two groups of chromosomes will have a
mixture of traits from the individual’s parents. Each gamete is unique.
Exercise:
Problem:
In what ways is meiosis II similar to and different from mitosis of a
diploid cell?
Solution:
The two divisions are similar in that the chromosomes line up along
the metaphase plate individually, meaning unpaired with other
chromosomes (as in meiosis I). In addition, each chromosome consists
of two sister chromatids that will be pulled apart. The two divisions are
different because in meiosis II there are half the number of
chromosomes that are present in a diploid cell of the same species
undergoing mitosis. This is because meiosis I reduced the number of
chromosomes to a haploid state.
Glossary
chiasmata
(singular = chiasma) the structure that forms at the crossover points
after genetic material is exchanged
crossing over
(also, recombination) the exchange of genetic material between
homologous chromosomes resulting in chromosomes that incorporate
genes from both parents of the organism forming reproductive cells
fertilization
the union of two haploid cells typically from two individual organisms
interkinesis
a period of rest that may occur between meiosis I and meiosis II; there
is no replication of DNA during interkinesis
meiosis |
the first round of meiotic cell division; referred to as reduction division
because the resulting cells are haploid
meiosis II
the second round of meiotic cell division following meiosis I; sister
chromatids are separated from each other, and the result is four unique
haploid cells
recombinant
describing something composed of genetic material from two sources,
such as a chromosome with both maternal and paternal segments of
DNA
reduction division
a nuclear division that produces daughter nuclei each having one-half
as many chromosome sets as the parental nucleus; meiosis I is a
reduction division
somatic cell
all the cells of a multicellular organism except the gamete-forming
cells
synapsis
the formation of a close association between homologous
chromosomes during prophase I
tetrad
two duplicated homologous chromosomes (four chromatids) bound
together by chiasmata during prophase I
Introduction to the Reproductive Systems
class="introduction"
Ovulation
Following
a surge of
luteinizin
g
hormone
(LH), an
oocyte
(immature
egg cell)
will be
released
into the
uterine
tube,
where it
will then
be
available
to be
fertilized
bya
male’s
sperm.
Ovulation
marks the
end of the
follicular
phase of
the
ovarian
cycle and
the start
of the
luteal
phase.
Note:
Chapter Objectives
After studying this chapter, you will be able to:
e Describe the anatomy of the male and female reproductive systems,
including their accessory structures
e Explain the role of hypothalamic and pituitary hormones in male and
female reproductive function
e Trace the path of a sperm cell from its initial production through
fertilization of an oocyte
e Explain the events in the ovary prior to ovulation
Small, uncoordinated, and slick with amniotic fluid, a newborn encounters
the world outside of her mother’s womb. We do not often consider that a
child’s birth is proof of the healthy functioning of both her mother’s and
father’s reproductive systems. Moreover, her parents’ endocrine systems
had to secrete the appropriate regulating hormones to induce the production
and release of unique male and female gametes, reproductive cells
containing the parents’ genetic material (one set of 23 chromosomes). Her
parent’s reproductive behavior had to facilitate the transfer of male gametes
—the sperm—to the female reproductive tract at just the right time to
encounter the female gamete, an oocyte (egg). Finally, combination of the
gametes (fertilization) had to occur, followed by implantation and
development. In this chapter, you will explore the male and female
reproductive systems, whose healthy functioning can culminate in the
powerful sound of a newborn’s first cry.
Male Reproductive Anatomy and Physiology
By the end of this section, you will be able to:
e Describe human male reproductive anatomy (structures and functions)
e Describe spermatogenesis a
e Describe the role of hormones in human reproduction
e Describe the roles of male reproductive hormones
Human Reproductive Anatomy
The reproductive tissues of male and female humans develop similarly in
utero until about the seventh week of gestation when a low level of the
hormone testosterone is released from the gonads of the developing male.
Testosterone causes the primitive gonads to differentiate into male sexual
organs. When testosterone is absent, the primitive gonads develop into
ovaries. Tissues that produce a penis in males produce a clitoris in females.
The tissue that will become the scrotum in a male becomes the labia in a
female. Thus the male and female anatomies arise from a divergence in the
development of what were once common embryonic structures.
Male Reproductive Anatomy
Proper development of sperm cells requires a temperature slightly lower
than the normal body temperature; therefore, the pair of testes must be
suspended outside the pelvic cavity (in the scrotum) so the environment of
the sperm is about 2 °C lower than body temperature. If the testes do not
descend through the abdominal cavity during fetal development, the
individual has reduced fertility.
The scrotum houses the testicles or testes (singular: testis), and provides
passage for blood vessels, nerves, and muscles related to testicular function.
The testes are a pair of male gonads that produce sperm and reproductive
hormones. Coiled in each testis are seminiferous tubules, where sperm
production begins.
The penis drains urine from the urinary bladder and is a copulatory organ
during intercourse ([link]; [link]). The penis contains three tubes of erectile
tissue that become engorged with blood, making the penis erect, in
preparation for intercourse. The organ is inserted into the vagina
culminating with an ejaculation. During orgasm, the accessory organs and
glands connected to the testes contract and empty the semen (containing
sperm) into the urethra and the fluid is expelled from the body by muscular
contractions causing ejaculation. After intercourse, the blood drains from
the erectile tissue and the penis becomes flaccid.
Semen is a mixture of sperm (about five percent of the total) and fluids
from accessory glands (prostate, bulbourethral glands, and seminal vesicles)
that contribute most of the semen’s volume. Sperm are haploid cells,
consisting of a flagellum for movement, a neck that contains the cell’s
energy-producing mitochondria, and a head that contains the genetic
material ({link]). An acrosome (acrosomal vesicle) is found at the top of the
head of the sperm. This structure contains enzymes that can digest the
protective coverings that surround the egg and allow the sperm to fuse with
the egg. An ejaculate will contain from two to five milliliters of fluid and
from 50—120 million sperm per milliliter.
As seen in this scanning electron
micrograph, human sperm has a
flagellum, neck, and head. (credit:
scale-bar data from Matt Russell)
Sperm cell formation begins in the walls of seminiferous tubules that are
coiled inside the testes ([link]; [link]). The walls of the seminiferous tubules
are made up of the developing sperm cells, with the least developed sperm
at the periphery of the tubule; the cells get pushed closer to the lumen as
maturation continues. The sperm cells are associated with Sertoli cells that
nourish and promote the development of the sperm. Other cells present
between the walls of the tubules are the Leydig/interstitial cells, which
produce testosterone once the male reaches puberty.
When the sperm have developed flagella they leave the seminiferous
tubules and enter the epididymis ([link]; [link]). This structure lies along the
top and back side of the testes and is the site of sperm maturation. The
sperm leave the epididymis and enter the vas deferens, which carries the
sperm behind the bladder, and forms the ejaculatory duct with the duct from
the seminal vesicles. During a vasectomy, a section of the vas deferens is
removed, preventing sperm (but not the secretions of the accessory glands)
from being passed out of the body during ejaculation and preventing
fertilization. Although a vasectomy is in many cases reversible via surgery,
it is still considered to be a permanent procedure.
The bulk of the semen comes from the accessory glands associated with the
male reproductive system. These are the seminal vesicles, the prostate
gland, and the bulbourethral gland ((link]; [link]). The secretions from
the accessory glands provide important compounds for the sperm including
nutrients, electrolytes, and pH buffering.
Note:
Art Connection
Pubic bone Bladder Seminal vesicle
Corpus Ejaculatory
cavernosum duct
Prostate
Corpus
spongiosum | gland
erie 6 cae Rectum
—~ff | . J Bulbourethral
Urethra — gland
Foreskin { ry Anus
Glans ~\ Vas deferens
Urethral Epididymis
Testis
Opening Scrotum
Seminiferous tubules
The reproductive structures of the human male are
shown.
Male Reproductive Anatomy
Organ Location Function
Supports testes and regulates
Scrotum External
their temperature
Penis External Delivers urine, copulating organ
Male Reproductive Anatomy
Organ Location Function
re invanaal Produce sperm and male
hormones
Seminal : ;
Internal Contribute to semen production
Vesicles
Prostate Gland Internal Contributes to semen production
Bulbourethtral a
orem Internal Neutralize urine in urethra
Glands
Gametogenesis: Spermatogenesis
Gametogenesis, the production of sperm and eggs, involves the process of
meiosis. During meiosis, two nuclear divisions separate the paired
chromosomes in the nucleus and then separate the chromatids that were
made during an earlier stage of the cell’s life cycle. Meiosis and its
associated cell divisions produces haploid (n) cells with half of each pair of
chromosomes normally found in diploid (2n)cells. The production of sperm
is called spermatogenesis.
Spermatogenesis
Spermatogenesis occurs in the wall of the seminiferous tubules, with the
most primitive cells at the periphery of the tube and the most mature sperm
at the lumen of the tube ([Llink]). Immediately under the capsule of the
tubule are diploid, undifferentiated cells. These stem cells, each called a
spermatogonium (pl. spermatogonia), go through mitosis to produce one
cell that remains as a stem cell and a second cell called a primary
spermatocyte that will undergo meiosis to produce sperm.
The diploid primary spermatocyte goes through meiosis I to produce two
haploid cells called secondary spermatocytes. Each secondary spermatocyte
divides after meiosis II to produce two cells called spermatids. The
spermatids eventually reach the lumen of the tubule and grow a flagellum,
becoming sperm cells. Four sperm result from each primary spermatocyte
that goes through meiosis.
Spermatogonium
CT | csi
@ Primary spermatocyte
| Meiosis |
(an) (an) Secondary spermatocyte
Meiosis Il
@ @ @ Onn
[soon
' t rr
<—_—
During spermatogenesis, four
sperm result from each
primary spermatocyte. The
process also maps onto the
physical structure of the wall
of the seminiferous tubule,
with the spermatogonia on
the outer side of the tubule,
and the sperm with their
developing tails extended
into the lumen of the tubule.
The process takes
approximately 70 days.
Note:
Concept in Action
ne
mess" OPenstax COLLEGE
Visit this site to see the process of spermatogenesis.
Hormonal Control of Reproduction
The human male and female reproductive cycles are controlled by the
interaction of hormones from the hypothalamus and anterior pituitary with
hormones from reproductive tissues and organs. In both sexes, the
hypothalamus monitors and causes the release of hormones from the
anterior pituitary gland. When the reproductive hormone is required, the
hypothalamus sends a gonadotropin-releasing hormone (GnRH) to the
anterior pituitary. This causes the release of follicle stimulating hormone
(FSH) and luteinizing hormone (LH) from the anterior pituitary into the
blood. Although these hormones are named after their functions in female
reproduction, they are produced in both sexes and play important roles in
controlling reproduction. Other hormones have specific functions in the
male and female reproductive systems.
Male Hormones
At the onset of puberty, the hypothalamus causes the release of FSH and
LH into the male system for the first time. FSH enters the testes and
stimulates the Sertoli cells located in the walls of the seminiferous tubules
to begin promoting spermatogenesis ([link]). LH also enters the testes and
stimulates the interstitial cells of Leydig, located in between the walls of the
seminiferous tubules, to make and release testosterone into the testes and
the blood.
Testosterone stimulates spermatogenesis. This hormone is also responsible
for the secondary sexual characteristics that develop in the male during
adolescence. The secondary sex characteristics in males include a
deepening of the voice, the growth of facial, axillary, and pubic hair, an
increase in muscle bulk, and the beginnings of the sex drive.
Pituitary hormone effects: [Hypothalamus |
LH and FSH stimulate spermatogenesis
and testosterone secretion by the testes. y GnRH
Inhibin
LH FSH Testosterone
Testes hormone effects:
Testosterone and inhibin inhibit
the secretion of GnRH by the
hypothalamus and LH and FSH
by the pituitary.
Sertoli cells facilitate
spermatogenesis
Leydig cells
Hormones control sperm production in a negative feedback system.
A negative feedback system occurs in the male with rising levels of
testosterone acting on the hypothalamus and anterior pituitary to inhibit the
release of GnRH, FSH, and LH. In addition, the Sertoli cells produce the
hormone inhibin, which is released into the blood when the sperm count is
too high. This inhibits the release of GnRH and FSH, which will cause
spermatogenesis to slow down. If the sperm count reaches a low of 20
million/mL, the Sertoli cells cease the release of inhibin, and the sperm
count increases.
Section Summary
The reproductive structures that evolved in land animals allow males and
females to mate, fertilize internally, and support the growth and
development of offspring. Gametogenesis, the production of sperm in the
male (spermatogenesis), takes place through the process of meiosis.
The male and female reproductive cycles are controlled by hormones
released from the hypothalamus and anterior pituitary and hormones from
reproductive tissues and organs. The hypothalamus monitors the need for
FSH and LH production and release from the anterior pituitary. FSH and
LH affect reproductive structures to cause the formation of sperm and the
preparation of eggs for release and possible fertilization. In the male, FSH
and LH stimulate Sertoli cells and interstitial cells of Leydig in the testes to
facilitate sperm production. The Leydig cells produce testosterone, which
also is responsible for the secondary sexual characteristics of males. In
females, FSH and LH cause estrogen and progesterone to be produced.
They regulate the female reproductive cycle, which is divided into the
ovarian cycle and the menstrual cycle.
Art Connections
Exercise:
Problem:
[link] Which of the following statements about the male reproductive
system is false?
a. The vas deferens carries sperm from the testes to the seminal
vesicles.
b. The ejaculatory duct joins the urethra.
c. Both the prostate and the bulbourethral glands produce
components of the semen.
d. The prostate gland is located in the testes.
Solution:
[link] D
Review Questions
Exercise:
Problem:Sperm are produced in the
a. scrotum
b. seminal vesicles
c. seminiferous tubules
d. prostate gland
Solution:
C
Exercise:
Problem: Which hormone causes FSH and LH to be released?
a. testosterone
b. estrogen
c. GnRH
d. progesterone
Solution:
Free Response
Exercise:
Problem:
Discuss spermatogenesis with respect to the timing of the process, and
the number and types of cells finally produced.
Solution:
Stem cells are laid down in the male during gestation and lie dormant
until adolescence/puberty. At this time, spermatogenesis begins and
continues until death, producing the maximum number of sperm (i.e.
four per cell that started meiosis) with each meiotic division. The
process takes approximately 70 days and the sperm are released into
the lumen of the seminiferous tubules.
Glossary
bulbourethral gland
the paired glands in the human male that produce a secretion that
cleanses the urethra prior to ejaculation
corpus luteum
the endocrine tissue that develops from an ovarian follicle after
ovulation; secretes progesterone and estrogen during pregnancy
clitoris
a sensory and erectile structure in female mammals, homologous to the
male penis, stimulated during sexual arousal
estrogen
a reproductive hormone in females that assists in endometrial
regrowth, ovulation, and calcium absorption
follicle stimulating hormone (FSH)
a reproductive hormone that causes sperm production in men and
follicle development in women
gestation
the development before birth of a viviparous animal
gestation period
the length of time of development, from conception to birth, of the
young of a viviparous animal
gonadotropin-releasing hormone (GnRH)
a hormone from the hypothalamus that causes the release of FSH and
LH from the anterior pituitary
human beta chorionic gonadotropin (§-HCG)
a hormone produced by the chorion of the zygote that helps to
maintain the corpus luteum and elevated levels of progesterone
inhibin
a hormone made by Sertoli cells, provides negative feedback to
hypothalamus in control of FSH and GnRH release
interstitial cell of Leydig
a cell type found next to the seminiferous tubules that makes
testosterone
labia majora
the large folds of tissue covering inguinal area
labia minora
the smaller folds of tissue within labia majora
luteinizing hormone (LH)
a reproductive hormone in both men and women, causes testosterone
production in men and ovulation and lactation in women
menstrual cycle
the cycle of the degradation and re-growth of the endometrium
oogenesis
the process of producing haploid eggs
ovarian cycle
the cycle of preparation of egg for ovulation and the conversion of the
follicle to the corpus luteum
oviduct
(also, fallopian tube) the muscular tube connecting uterus with ovary
area
ovulation
the release of an oocyte from a mature follicle in the ovary of a
vertebrate
penis
the male reproductive structure for urine elimination and copulation
placenta
the organ that supports the transport of nutrients and waste between the
mothers and fetus’ blood in eutherian mammals
progesterone
a reproductive hormone in women; assists in endometrial regrowth and
inhibition of FSH and LH release
prostate gland
a structure that is a mixture of smooth muscle and glandular material
and that contributes to semen
scrotum
a sac containing testes, exterior to body
semen
a fluid mixture of sperm and supporting materials
seminal vesicle
a secretory accessory gland in male; contributes to semen
seminiferous tubule
the structures within which sperm production occurs in the testes
Sertoli cell
a cell in the walls of the seminiferous tubules that assists developing
sperm and secretes inhibin
spermatogenesis
the process of producing haploid sperm
testes
a pair of male reproductive organs
testosterone
a reproductive hormone in men that assists in sperm production and
promoting secondary sexual characteristics
uterus
a female reproductive structure in which an embryo develops
vagina
a muscular tube for the passage of menstrual flow, copulation, and
birth of offspring
Female Reproductive Anatomy and Physiology; Gestation and Labor
By the end of this section, you will be able to:
e Describe human female reproductive anatomy
¢ Describe oogenesis and discuss its differences and similarities to
spermatogenesis
e Describe the roles of female reproductive hormones
e Describe major events of gestation and labor
Human Reproductive Anatomy
Female Reproductive Anatomy
A number of female reproductive structures are exterior to the body. These
include the breasts and the vulva, which consists of the mons pubis, clitoris,
and labia. ([link]; [link]).
Ovaries
Uterus
Bladder
Cervix aa
Fimbrae
Vagina
Urethra
Clitoris
Labium minora
Labium majora Vagina
(a) (b)
The reproductive structures of the human female are shown.
(credit a: modification of work by Gray's Anatomy; credit b:
modification of work by CDC)
The breasts consist of mammary glands and fat. Each gland consists of 15
to 25 lobes that have ducts that empty at the nipple and that supply the
nursing child with nutrient- and antibody-rich milk to aid development and
protect the child.
Internal female reproductive structures include ovaries, oviducts, the uterus,
and the vagina ([link]; [link]). The pair of ovaries is held in place in the
abdominal cavity by a system of ligaments. The outermost layer of the
ovary is made up of follicles that surround, nourish, and protect a single
egg. During the menstrual period, a batch of follicles develops and prepares
their eggs for release. At ovulation, one follicle ruptures and one egg is
released. Following ovulation, the follicular tissue that surrounded the
ovulated egg stays within the ovary and grows to form a solid mass called
the corpus luteum. The corpus luteum secretes additional estrogen and the
hormone progesterone that helps maintain the uterine lining during
pregnancy. The ovaries also produce hormones, such as estrogen.
The oviducts, or fallopian tubes, extend from the uterus in the lower
abdominal cavity to the ovaries, but they are not in contact with the ovaries.
The lateral ends of the oviducts flare out into a trumpet-like structure and
have a fringe of finger-like projections called fimbrae. When an egg is
released at ovulation, the fimbrae help the nonmotile egg enter into the
tube. The walls of the oviducts have a ciliated epithelium over smooth
muscle. The cilia beat, and the smooth muscle contracts, moving the egg
toward the uterus. Fertilization usually takes place within the oviduct and
the developing embryo is moved toward the uterus. It usually takes the egg
or embryo a week to travel through the oviduct.
Sterilization in women is called a tubal ligation; it is analogous to a
vasectomy in males in that the oviducts are severed and sealed, preventing
sperm from reaching the egg.
The uterus is a structure about the size of a woman’s fist. The uterus has a
thick muscular wall and is lined with an endometrium rich in blood vessels
and mucus glands that develop and thicken during the female cycle.
Thickening of the endometrium prepares the uterus to receive the fertilized
egg or zygote, which will then implant itself in the endometrium. The
uterus supports the developing embryo and fetus during gestation.
Contractions of the smooth muscle in the uterus aid in forcing the baby
through the vagina during labor. If fertilization does not occur, a portion of
the lining of the uterus sloughs off during each menstrual period. The
endometrium builds up again in preparation for implantation. Part of the
uterus, called the cervix, protrudes into the top of the vagina.
The vagina is a muscular tube that serves several purposes. It allows
menstrual flow to leave the body. It is the receptacle for the penis during
intercourse and the pathway for the delivery of offspring.
Female Reproductive Anatomy
Organ Location Function
Clitoris External Sensory organ
pons External Fatty area overlying pubic bone
pubis
Breast External Produces and delivers milk
Ovaries Internal Produce and develop eggs
Ovidueks ee Transport egg to uterus; site of
fertilization
Uterus Internal Supports developing embryo
Vagina inital Common tube for intercourse, birth
canal, passing menstrual flow
Gametogenesis (Oogenesis)
Gametogenesis, the production of sperm and eggs, involves the process of
meiosis. During meiosis, two nuclear divisions separate the paired
chromosomes in the nucleus and then separate the chromatids that were
made during an earlier stage of the cell’s life cycle. Meiosis and its
associated cell divisions produces haploid (n) cells with half of each pair of
chromosomes normally found in diploid (2n) cells. The production of sperm
is called spermatogenesis and the production of eggs is called oogenesis.
Oogenesis
Oogenesis occurs in the outermost layers of the ovaries. As with sperm
production, oogenesis starts with a germ cell. In oogenesis, this germ cell is
called an oogonium and forms during the embryological development of the
individual. The oogonium undergoes mitosis to produce about one to two
million oocytes by the time of birth.
(2n) Oogonium
|e
. Primary oocyte
Before birth (arrests in prophase 1)
Pitot puberty | meta continues
Secondary oocyte
Polar body @) (an) (arrests in metaphase I!)
[oni sperm entry
~\jp—@
| mn fertilization
Polar body da (2n) Fertilized Egg
The process of oogenesis
occurs in the ovary’s
outermost layer.
The primary oocytes begin meiosis before birth ([link]). However, the
meiotic division is arrested in its progress in the first prophase stage. At the
time of birth, all future eggs are in prophase I. This situation is in contrast
with the male reproductive system in which sperm are produced
continuously throughout the life of the individual. Starting at adolescence,
anterior pituitary hormones cause the development of a few follicles in an
ovary each month. This results in a primary oocyte finishing the first
meiotic division. The cell divides unequally, with most of the cytoplasm
and organelles going to one cell, called a secondary oocyte, and only one
set of chromosomes and a small amount of cytoplasm going to the other
cell. This second cell is called a polar body and usually dies. Cell division is
again arrested, this time at metaphase II. At ovulation, this secondary
oocyte is released and travels toward the uterus through the oviduct. If the
secondary oocyte is fertilized, the cell continues through meiosis II,
producing a second polar body and haploid egg, which fuses with the
haploid sperm to form a fertilized egg (zygote) containing all 46
chromosomes.
Hormonal Control of Reproduction
The human male and female reproductive cycles are controlled by the
interaction of hormones from the hypothalamus and anterior pituitary with
hormones from reproductive tissues and organs. In both sexes, the
hypothalamus monitors and causes the release of hormones from the
anterior pituitary gland. When the reproductive hormone is required, the
hypothalamus sends a gonadotropin-releasing hormone (GnRH) to the
anterior pituitary. This causes the release of follicle stimulating hormone
(FSH) and luteinizing hormone (LH) from the anterior pituitary into the
blood. Although these hormones are named after their functions in female
reproduction, they are produced in both sexes and play important roles in
controlling reproduction. Other hormones have specific functions in the
male and female reproductive systems.
Female Hormones
The control of reproduction in females is more complex that in males. The
female reproductive cycle is divided into the ovarian cycle and the
menstrual cycle. The ovarian cycle governs the preparation of endocrine
tissues and release of eggs, while the menstrual cycle governs the
preparation and maintenance of the uterine lining ([link]). These cycles are
coordinated over a 22—32 day cycle, with an average length of 28 days.
As with the male, the GnRH from the hypothalamus causes the release of
the hormones FSH and LH from the anterior pituitary. In addition, estrogen
and relatively small amounts of progesterone are released from the
developing follicles. As with testosterone in males, estrogen is responsible
for the secondary sexual characteristics of females. These include breast
development, flaring of the hips, and a shorter period for bone growth.
The Ovarian Cycle and the Menstrual Cycle
The ovarian and menstrual cycles are regulated by hormones of the
hypothalamus, pituitary, and ovaries ([link]). The ebb and flow of the
hormones causes the ovarian and menstrual cycles to advance. The ovarian
and menstrual cycles occur concurrently. The first half of the ovarian cycle
is the follicular phase. Slowly rising levels of FSH cause the growth of
follicles on the surface of the ovary. This process prepares the egg for
ovulation. As the follicles grow, they begin releasing estrogen. The first few
days of this cycle coincide with menstruation or the sloughing off of the
functional layer of the endometrium in the uterus. After about five days,
estrogen levels rise and the menstrual cycle enters the proliferative phase.
The endometrium begins to regrow, replacing the blood vessels and glands
that deteriorated during the end of the last cycle.
Note:
Art Connection
| Follicular phase Il Ovulation
Pituitary hormone Pituitary hormone
effect: LH and FSH effect: LH and FSH
stimulate several stimulate maturation GnRH
follicles to grow. of one of the
growing follicles.
Estrogen
Ovarian Ovarian
hormone hormone
effects: effects:
Follicles produce
low levels of
estrogen that
e Inhibit GnRH
secretion by
| Fotices | the hypothalamus,
keeping LH and
: FSH
LH
FSH levels low.
Estrogen e Cause endometrial Estrogen levels rise,
arteries to constrict resulting in
due to low levels, ovulation about
resulting in a day later.
Endometum menstruation. JEndometum e Cause the
endometrium
to thicken.
Dominant follicle
produces high
levels of estrogen,
which:
e Stimulate GnRH
secretion by the
hypothalamus.
LH and FSH
LH
Estrogen
Ill Luteal phase
Pituitary hormone
effect: LH stimulates
formation of a corpus GnRH
luteum from follicular
tissue left behind after
ovulation.
Ovarian
hormone
effects:
Estrogen The corpus
luteum secretes
Progesterone estrogen and
;
progesterone that
Corpus luteum e Block GnRH
production by the
hypothalamus
Estrogen and LH and
! FSH production
Progesterone by the pituitary.
ale Cause the
endometrium endometrium to
further develop.
The ovarian and menstrual cycles of female reproduction are
regulated by hormones produced by the hypothalamus, pituitary, and
ovaries.
Just prior to the middle of the cycle (approximately day 14), the high level
of estrogen causes FSH and especially LH to rise rapidly then fall. The
spike in LH causes the most mature follicle to rupture and release its egg.
This is ovulation. The follicles that did not rupture degenerate and their
eggs are lost. The level of estrogen decreases when the extra follicles
degenerate.
Following ovulation, the ovarian cycle enters its luteal phase and the
menstrual cycle enters its secretory phase, both of which run from about
day 15 to 28. The luteal and secretory phases refer to changes in the
ruptured follicle. The cells in the follicle undergo physical changes and
produce a structure called a corpus luteum. The corpus luteum produces
estrogen and larger amounts of progesterone. The progesterone facilitates
the regrowth of the uterine lining and inhibits the release of further FSH and
LH. The uterus is being prepared to accept a fertilized egg, should it occur
during this cycle. The inhibition of FSH and LH prevents any further eggs
and follicles from developing, while the progesterone is elevated. The level
of estrogen produced by the corpus luteum increases to a steady level for
the next few days.
If no fertilized egg is implanted into the uterus, the corpus luteum
degenerates and the levels of estrogen and progesterone decrease. The
endometrium begins to degenerate as the progesterone levels drop,
initiating the next menstrual cycle. The decrease in progesterone also allows
the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and
LH and starting the cycles again.
Note:
Career in Action
Reproductive Endocrinologist
A reproductive endocrinologist is a physician who treats a variety of
hormonal disorders related to reproduction and infertility in both men and
women. The disorders include menstrual problems, infertility, pregnancy
loss, sexual dysfunction, and menopause. Doctors may use fertility drugs,
surgery, or assisted reproductive techniques (ART) in their therapy. ART
involves the use of procedures to manipulate the egg or sperm to facilitate
reproduction, such as in vitro fertilization.
Reproductive endocrinologists undergo extensive medical training, first in
a four-year residency in obstetrics and gynecology, then in a three-year
fellowship in reproductive endocrinology. To be board certified in this
area, the physician must pass written and oral exams in both areas.
Gestation
Pregnancy begins with the fertilization of an egg and continues through to
the birth of the individual. The length of time of gestation, or the gestation
period, in humans is approximately 266 days.
Within 24 hours of fertilization, the egg nucleus has finished meiosis and
the egg and sperm nuclei fuse. With fusion, the cell is known as a zygote.
The zygote initiates cleavage and the developing embryo travels through
the oviduct to the uterus. The developing embryo must implant into the wall
of the uterus within seven days, or it will deteriorate and die. The outer
layers of the developing embryo or blastocyst grow into the endometrium
by digesting the endometrial cells, and healing of the endometrium closes
up the blastocyst into the tissue. Another layer of the blastocyst, the
chorion, begins releasing a hormone called human beta chorionic
gonadotropin (B-HCG), which makes its way to the corpus luteum and
keeps that structure active. This ensures adequate levels of progesterone
that will maintain the endometrium of the uterus for the support of the
developing embryo. Pregnancy tests determine the level of B-HCG in urine
or serum. If the hormone is present, the test is positive.
The gestation period is divided into three equal periods or trimesters.
During the first two-to-four weeks of the first trimester, nutrition and waste
are handled by the endometrial lining through diffusion. As the trimester
progresses, the outer layer of the embryo begins to merge with the
endometrium, and the placenta forms. The placenta takes over the nutrient
and waste requirements of the embryo and fetus, with the mother’s blood
passing nutrients to the placenta and removing waste from it. Chemicals
from the fetus, such as bilirubin, are processed by the mother’s liver for
elimination. Some of the mother’s immunoglobulins will pass through the
placenta, providing passive immunity against some potential infections.
Internal organs and body structures begin to develop during the first
trimester. By five weeks, limb buds, eyes, the heart, and liver have been
basically formed. By eight weeks, the term fetus applies, and the body is
essentially formed ({link]a). The individual is about five centimeters (two
inches) in length and many of the organs, such as the lungs and liver, are
not yet functioning. Exposure to any toxins is especially dangerous during
the first trimester, as all of the body’s organs and structures are going
through initial development. Anything that interferes with chemical
signaling during that development can have a severe effect on the fetus’
survival.
Umbilical cord
(b) (c)
(a) Fetal development is shown at nine weeks
gestation. (b) This fetus is just entering the second
trimester, when the placenta takes over more of the
functions performed as the baby develops. (c)
There is rapid fetal growth during the third
trimester. (credit a: modification of work by Ed
Uthman; credit b: modification of work by
National Museum of Health and Medicine; credit
c: modification of work by Gray’s Anatomy)
During the second trimester, the fetus grows to about 30 cm (about 12
inches) ([link]|b). It becomes active and the mother usually feels the first
movements. All organs and structures continue to develop. The placenta has
taken over the functions of nutrition and waste elimination and the
production of estrogen and progesterone from the corpus luteum, which has
degenerated. The placenta will continue functioning up through the delivery
of the baby. During the third trimester, the fetus grows to 3 to 4 kg (6.5-8.5
Ibs.) and about 50 cm (19-20 inches) long ([link]c). This is the period of the
most rapid growth during the pregnancy as all organ systems continue to
grow and develop.
Labor is the muscular contractions to expel the fetus and placenta from the
uterus. Toward the end of the third trimester, estrogen causes receptors on
the uterine wall to develop and bind the hormone oxytocin. At this time, the
baby reorients, facing forward and down with the back or crown of the head
engaging the cervix (uterine opening). This causes the cervix to stretch and
nerve impulses are sent to the hypothalamus, which signals the release of
oxytocin from the posterior pituitary. Oxytocin causes smooth muscle in the
uterine wall to contract. At the same time, the placenta releases
prostaglandins into the uterus, increasing the contractions. A positive
feedback relay occurs between the uterus, hypothalamus, and the posterior
pituitary to assure an adequate supply of oxytocin. As more smooth muscle
cells are recruited, the contractions increase in intensity and force.
There are three stages to labor. During stage one, the cervix thins and
dilates. This is necessary for the baby and placenta to be expelled during
birth. The cervix will eventually dilate to about 10 cm. During stage two,
the baby is expelled from the uterus. The uterus contracts and the mother
pushes as she compresses her abdominal muscles to aid the delivery. The
last stage is the passage of the placenta after the baby has been born and the
organ has completely disengaged from the uterine wall. If labor should stop
before stage two is reached, synthetic oxytocin, known as Pitocin, can be
administered to restart and maintain labor.
Section Summary
The female reproductive cycle is controlled by hormones released from the
hypothalamus and anterior pituitary and hormones from reproductive
tissues and organs. The hypothalamus monitors the need for FSH and LH
production and release from the anterior pituitary. FSH and LH affect
reproductive structures to cause the preparation of eggs for release and
possible fertilization. In females, FSH and LH cause estrogen and
progesterone to be produced. They regulate the female reproductive cycle,
which is divided into the ovarian cycle and the menstrual cycle.
Human pregnancy begins with fertilization of an egg and proceeds through
the three trimesters of gestation. The first trimester lays down the basic
structures of the body, including the limb buds, heart, eyes, and the liver.
The second trimester continues the development of all of the organs and
systems. The third trimester exhibits the greatest growth of the fetus and
culminates in labor and delivery. The labor process has three stages
(contractions, delivery of the fetus, and expulsion of the placenta), each
propelled by hormones.
Art Connections
Exercise:
Problem:
[link] Which of the following statements about hormone regulation of
the female reproductive cycle is false?
a. LH and FSH are produced in the pituitary, and estrogen and
progesterone are produced in the ovaries.
b. Estrogen and progesterone secreted from the corpus luteum (CL)
cause the endometrium to thicken.
c. Follicles produce high levels of progesterone.
d. Secretion of GnRH by the hypothalamus is inhibited by high
levels of estrogen.
Solution:
[link] C
Review Questions
Exercise:
Problem:
Which female organ has an endometrial lining that will support a
developing baby?
a. labia minora
b. breast
c. Ovaries
d. uterus
Solution:
D
Exercise:
Problem: Which hormone causes FSH and LH to be released?
a. testosterone
b. estrogen
c. GnRH
d. progesterone
Solution:
@
Exercise:
Problem:
Nutrient and waste requirements for the developing fetus are handled
during the first few weeks by
a. the placenta
b. diffusion through the endometrium
c. the chorion
d. the blastocyst
Solution:
B
Exercise:
Problem:
Which hormone is primarily responsible for the contractions during
labor?
a. oxytocin
b. estrogen
c. B-HCG
d. progesterone
Solution:
A
Free Response
Exercise:
Problem:
Describe oogenesis with respect to the timing of the processes and the
number and type of cells finally produced.
Solution:
Stem cells in the female increase to one to two million and enter the
first meiotic division and are arrested in prophase. Oogenesis
continues again at adolescence in batches of eggs with each menstrual
cycle. These primary oocytes finish the first meiotic division,
producing a viable egg with most of the cytoplasm and its contents,
and a second cell called a polar body containing 23 chromosomes. The
second meiotic division is initiated and arrested in metaphase. At
ovulation, one egg is released. If this egg is fertilized, it finishes the
second meiotic division. This is a diploid, fertilized egg.
Exercise:
Problem:
Describe the events in the ovarian cycle leading up to ovulation.
Solution:
Low levels of progesterone allow the hypothalamus to send GnRH to
the anterior pituitary and cause the release of FSH and LH. FSH
stimulates follicles on the ovary to grow and prepare the eggs for
ovulation. As the follicles increase in size, they begin to release
estrogen and a low level of progesterone into the blood. The level of
estrogen rises to a peak, causing a spike in the concentration of LH.
This causes the most mature follicle to rupture and ovulation occurs.
Exercise:
Problem: Describe the stages of labor.
Solution:
Stage one of labor results in uterine contractions, which thin the cervix
and dilate the cervical opening. Stage two delivers the baby, and stage
three delivers the placenta.
Glossary
bulbourethral gland
the paired glands in the human male that produce a secretion that
cleanses the urethra prior to ejaculation
corpus luteum
the endocrine tissue that develops from an ovarian follicle after
ovulation; secretes progesterone and estrogen during pregnancy
clitoris
a sensory and erectile structure in female mammals, homologous to the
male penis, stimulated during sexual arousal
estrogen
a reproductive hormone in females that assists in endometrial
regrowth, ovulation, and calcium absorption
follicle stimulating hormone (FSH)
a reproductive hormone that causes sperm production in men and
follicle development in women
gestation
the development before birth of a viviparous animal
gestation period
the length of time of development, from conception to birth, of the
young of a viviparous animal
gonadotropin-releasing hormone (GnRH)
a hormone from the hypothalamus that causes the release of FSH and
LH from the anterior pituitary
human beta chorionic gonadotropin (§-HCG)
a hormone produced by the chorion of the zygote that helps to
maintain the corpus luteum and elevated levels of progesterone
inhibin
a hormone made by Sertoli cells, provides negative feedback to
hypothalamus in control of FSH and GnRH release
interstitial/Leydig cell
a cell type found next to the seminiferous tubules that makes
testosterone
luteinizing hormone (LH)
a reproductive hormone in both men and women, causes testosterone
production in men and ovulation and lactation in women
menstrual cycle
the cycle of the degradation and re-growth of the endometrium
oogenesis
the process of producing haploid eggs
ovarian cycle
the cycle of preparation of egg for ovulation and the conversion of the
follicle to the corpus luteum
oviduct
(also, fallopian tube) the muscular tube connecting uterus with ovary
area
ovulation
the release of an oocyte from a mature follicle in the ovary of a
vertebrate
penis
the male reproductive structure for urine elimination and copulation
placenta
the organ that supports the transport of nutrients and waste between the
mothers and fetus’ blood in eutherian mammals
progesterone
a reproductive hormone in women; assists in endometrial regrowth and
inhibition of FSH and LH release
prostate gland
a structure that is a mixture of smooth muscle and glandular material
and that contributes to semen
scrotum
a sac containing testes, exterior to body
semen
a fluid mixture of sperm and supporting materials
seminal vesicle
a secretory accessory gland in male; contributes to semen
seminiferous tubule
the structures within which sperm production occurs in the testes
Sertoli cell
a cell in the walls of the seminiferous tubules that assists developing
sperm and secretes inhibin
spermatogenesis
the process of producing haploid sperm
testes
a pair of male reproductive organs
testosterone
a reproductive hormone in men that assists in sperm production and
promoting secondary sexual characteristics
uterus
a female reproductive structure in which an embryo develops
vagina
a muscular tube for the passage of menstrual flow, copulation, and
birth of offspring
Introduction to Bone Tissue
class="introduction"
Child Looking at Bones
Bone is a
living tissue.
Unlike the
bones of a
fossil made
inert by a
process of
mineralization
, achild’s
bones will
continue to
grow and
develop while
contributing to
the support
and function
of other body
systems.
(credit: James
Emery)
FOSSIL CROCODILE
Azraripesuchus pa
Note:
Chapter Objectives
After studying this chapter, you will be able to:
List and describe the functions of bones
Describe the classes of bones
e Discuss the process of bone formation and development
e Explain how bone repairs itself after a fracture
e Discuss the effect of exercise, nutrition, and hormones on bone tissue
Bones make good fossils. While the soft tissue of a once living organism
will decay and fall away over time, bone tissue will, under the right
conditions, undergo a process of mineralization, effectively turning the
bone to stone. A well-preserved fossil skeleton can give us a good sense of
the size and shape of an organism, just as your skeleton helps to define your
size and shape. Unlike a fossil skeleton, however, your skeleton is a
structure of living tissue that grows, repairs, and renews itself. The bones
within it are dynamic and complex organs that serve a number of important
functions, including some necessary to maintain homeostasis.
Functions of the Skeletal System
By the end of this section, you will be able to:
¢ Define bone, cartilage, and the skeletal system
e List and describe the functions of the skeletal system
Bone, or osseous tissue, is a hard, dense connective tissue that forms most
of the adult skeleton, the support structure of the body. In the areas of the
skeleton where bones move (for example, the ribcage and joints), cartilage,
a semi-rigid form of connective tissue, provides flexibility and smooth
surfaces for movement. The skeletal system is the body system composed
of bones and cartilage and performs the following critical functions for the
human body:
e supports the body
e facilitates movement
¢ protects internal organs
e produces blood cells
e stores and releases minerals and fat
Support, Movement, and Protection
The most apparent functions of the skeletal system are the gross functions
—those visible by observation. Simply by looking at a person, you can see
how the bones support, facilitate movement, and protect the human body.
Just as the steel beams of a building provide a scaffold to support its weight,
the bones and cartilage of your skeletal system compose the scaffold that
supports the rest of your body. Without the skeletal system, you would be a
limp mass of organs, muscle, and skin.
Bones also facilitate movement by serving as points of attachment for your
muscles. While some bones only serve as a support for the muscles, others
also transmit the forces produced when your muscles contract. From a
mechanical point of view, bones act as levers and joints serve as fulcrums
({link]). Unless a muscle spans a joint and contracts, a bone is not going to
move. For information on the interaction of the skeletal and muscular
systems, that is, the musculoskeletal system, seek additional content.
Bones Support Movement
Bones act as levers when
muscles span a joint and
contract. (credit: Benjamin J.
DeLong)
Bones also protect internal organs from injury by covering or surrounding
them. For example, your ribs protect your lungs and heart, the bones of
your vertebral column (spine) protect your spinal cord, and the bones of
your cranium (skull) protect your brain ({link]).
Bones Protect Brain
The cranium completely
surrounds and protects
the brain from non-
traumatic injury.
Note:
Career Connection
Orthopedist
An orthopedist is a doctor who specializes in diagnosing and treating
disorders and injuries related to the musculoskeletal system. Some
orthopedic problems can be treated with medications, exercises, braces,
and other devices, but others may be best treated with surgery ([link]).
Arm Brace
An orthopedist will sometimes prescribe
the use of a brace that reinforces the
underlying bone structure it is being used
to support. (credit: Juhan Sonin)
While the origin of the word “orthopedics” (ortho- = “straight”; paed- =
“child”), literally means “straightening of the child,” orthopedists can have
patients who range from pediatric to geriatric. In recent years, orthopedists
have even performed prenatal surgery to correct spina bifida, a congenital
defect in which the neural canal in the spine of the fetus fails to close
completely during embryologic development.
Orthopedists commonly treat bone and joint injuries but they also treat
other bone conditions including curvature of the spine. Lateral curvatures
(scoliosis) can be severe enough to slip under the shoulder blade (scapula)
forcing it up as a hump. Spinal curvatures can also be excessive
dorsoventrally (kyphosis) causing a hunch back and thoracic compression.
These curvatures often appear in preteens as the result of poor posture,
abnormal growth, or indeterminate causes. Mostly, they are readily treated
by orthopedists. As people age, accumulated spinal column injuries and
diseases like osteoporosis can also lead to curvatures of the spine, hence
the stooping you sometimes see in the elderly.
Some orthopedists sub-specialize in sports medicine, which addresses both
simple injuries, such as a sprained ankle, and complex injuries, such as a
torn rotator cuff in the shoulder. Treatment can range from exercise to
surgery.
Mineral Storage, Energy Storage, and Hematopoiesis
On a metabolic level, bone tissue performs several critical functions. For
one, the bone matrix acts as a reservoir for a number of minerals important
to the functioning of the body, especially calcium, and potassium. These
minerals, incorporated into bone tissue, can be released back into the
bloodstream to maintain levels needed to support physiological processes.
Calcium ions, for example, are essential for muscle contractions and
controlling the flow of other ions involved in the transmission of nerve
impulses.
Bone also serves as a site for fat storage and blood cell production. The
softer connective tissue that fills the interior of most bone is referred to as
bone marrow ((link]). There are two types of bone marrow: yellow marrow
and red marrow. Yellow marrow contains adipose tissue; the triglycerides
stored in the adipocytes of the tissue can serve as a source of energy. Red
marrow is where hematopoiesis—the production of blood cells—takes
place. Red blood cells, white blood cells, and platelets are all produced in
the red marrow.
Head of Femur Showing Red and Yellow Marrow
Outer surface of bone
Red marrow
Yellow marrow
The head of the femur contains both
yellow and red marrow. Yellow marrow
stores fat. Red marrow is responsible for
hematopoiesis. (credit: modification of
work by “stevenfruitsmaak”/Wikimedia
Commons)
Chapter Review
The major functions of the bones are body support, facilitation of
movement, protection of internal organs, storage of minerals and fat, and
hematopoiesis. Together, the muscular system and skeletal system are
known as the musculoskeletal system.
Review Questions
Exercise:
Problem:
Which function of the skeletal system would be especially important if
you were in a car accident?
a. storage of minerals
b. protection of internal organs
c. facilitation of movement
d. fat storage
Solution:
B
Exercise:
Problem:Bone tissue can be described as
a. dead calcified tissue
b. cartilage
c. the skeletal system
d. dense, hard connective tissue
Solution:
D
Exercise:
Problem: Without red marrow, bones would not be able to
a. store phosphate
b. store calcium
c. make blood cells
d. move like levers
Solution:
C
Exercise:
Problem: Yellow marrow has been identified as
a. an area of fat storage
b. a point of attachment for muscles
c. the hard portion of bone
d. the cause of kyphosis
Solution:
A
Exercise:
Problem: Which of the following can be found in areas of movement?
a. hematopoiesis
b. cartilage
c. yellow marrow
d. red marrow
Solution:
B
Exercise:
Problem:The skeletal system is made of
a. muscles and tendons
b. bones and cartilage
c. vitreous humor
d. minerals and fat
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
The skeletal system is composed of bone and cartilage and has many
functions. Choose three of these functions and discuss what features of
the skeletal system allow it to accomplish these functions.
Solution:
It supports the body. The rigid, yet flexible skeleton acts as a
framework to support the other organs of the body.
It facilitates movement. The movable joints allow the skeleton to
change shape and positions; that is, move.
It protects internal organs. Parts of the skeleton enclose or partly
enclose various organs of the body including our brain, ears, heart, and
lungs. Any trauma to these organs has to be mediated through the
skeletal system.
It produces blood cells. The central cavity of long bones is filled with
marrow. The red marrow is responsible for forming red and white
blood cells.
It stores and releases minerals and fat. The mineral component of
bone, in addition to providing hardness to bone, provides a mineral
reservoir that can be tapped as needed. Additionally, the yellow
marrow, which is found in the central cavity of long bones along with
red marrow, serves as a Storage site for fat.
Glossary
bone
hard, dense connective tissue that forms the structural elements of the
skeleton
cartilage
semi-rigid connective tissue found on the skeleton in areas where
flexibility and smooth surfaces support movement
hematopoiesis
production of blood cells, which occurs in the red marrow of the bones
orthopedist
doctor who specializes in diagnosing and treating musculoskeletal
disorders and injuries
osseous tissue
bone tissue; a hard, dense connective tissue that forms the structural
elements of the skeleton
red marrow
connective tissue in the interior cavity of a bone where hematopoiesis
takes place
skeletal system
organ system composed of bones and cartilage that provides for
movement, support, and protection
yellow marrow
connective tissue in the interior cavity of a bone where fat is stored
Bone Structure
By the end of this section, you will be able to:
e Identify the anatomical features of a bone
¢ Describe the histology of bone tissue
e¢ Compare and contrast compact and spongy bone
e Identify the structures that compose compact and spongy bone
e Describe how bones are nourished and innervated
Bone tissue (osseous tissue) differs greatly from other tissues in the body.
Bone is hard and many of its functions depend on that characteristic
hardness. Later discussions in this chapter will show that bone is also
dynamic in that its shape adjusts to accommodate stresses. This section will
examine the gross anatomy of bone first and then move on to its histology.
Gross Anatomy of Bone
The structure of a long bone allows for the best visualization of all of the
parts of a bone ([link]). A long bone has two parts: the diaphysis and the
epiphysis. The diaphysis is the tubular shaft that runs between the proximal
and distal ends of the bone. The hollow region in the diaphysis is called the
medullary cavity, which is filled with yellow marrow. The walls of the
diaphysis are composed of dense and hard compact bone.
Anatomy of a Long Bone
Articular cartilage
Proximal
epiphysis
Metaphysis Spongy bone
Epiphyseal line
Red bone marrow
Endosteum
Compact bone
Medullary cavity
Diaphysis Yellow bone marrow
Periosteum
Nutrient artery
Distal
epiphysis
Articular cartilage
A typical long bone shows the
gross anatomical characteristics of
bone.
The wider section at each end of the bone is called the epiphysis (plural =
epiphyses), which is filled with spongy bone. Red marrow fills the spaces in
the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the
narrow area that contains the epiphyseal plate (growth plate), a layer of
hyaline (transparent) cartilage in a growing bone. When the bone stops
growing in early adulthood (approximately 18-21 years), the cartilage is
replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal
line.
Bone Cells and Tissue
Bone contains a relatively small number of cells entrenched in a matrix of
collagen fibers that provide a surface for inorganic salt crystals to adhere.
These salt crystals form when calcium phosphate and calcium carbonate
combine to create hydroxyapatite, which incorporates other inorganic salts
like magnesium hydroxide, fluoride, and sulfate as it crystallizes, or
calcifies, on the collagen fibers. The hydroxyapatite crystals give bones
their hardness and strength, while the collagen fibers give them flexibility
so that they are not brittle.
Although bone cells compose a small amount of the bone volume, they are
crucial to the function of bones. Four types of cells are found within bone
tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts ([link]).
Bone Cells
Osteocyte Osteoblast Osteogenic cell Osteoclast
(maintains (forms bone matrix) (stem cell) (resorbs bone)
bone tissue)
Four types of cells are found within bone
tissue. Osteogenic cells are undifferentiated
and develop into osteoblasts. When
osteoblasts get trapped within the calcified
matrix, their structure and function changes,
and they become osteocytes. Osteoclasts
develop from monocytes and macrophages
and differ in appearance from other bone
cells.
The osteoblast is the bone cell responsible for forming new bone and is
found in the growing portions of bone, including the periosteum and
endosteum. Osteoblasts, which do not divide, synthesize and secrete the
collagen matrix and calcium salts. As the secreted matrix surrounding the
osteoblast calcifies, the osteoblast become trapped within it; as a result, it
changes in structure and becomes an osteocyte, the primary cell of mature
bone and the most common type of bone cell. Each osteocyte is located in a
space called a lacuna and is surrounded by bone tissue. Osteocytes
maintain the mineral concentration of the matrix via the secretion of
enzymes. Like osteoblasts, osteocytes lack mitotic activity. They can
communicate with each other and receive nutrients via long cytoplasmic
processes that extend through canaliculi (singular = canaliculus), channels
within the bone matrix.
If osteoblasts and osteocytes are incapable of mitosis, then how are they
replenished when old ones die? The answer lies in the properties of a third
category of bone cells—the osteogenic cell. These osteogenic cells are
undifferentiated with high mitotic activity and they are the only bone cells
that divide. Immature osteogenic cells are found in the deep layers of the
periosteum and the marrow. They differentiate and develop into osteoblasts.
The dynamic nature of bone means that new tissue is constantly formed,
and old, injured, or unnecessary bone is dissolved for repair or for calcium
release. The cell responsible for bone resorption, or breakdown, is the
osteoclast. They are found on bone surfaces, are multinucleated, and
originate from monocytes and macrophages, two types of white blood cells,
not from osteogenic cells. Osteoclasts are continually breaking down old
bone while osteoblasts are continually forming new bone. The ongoing
balance between osteoblasts and osteoclasts is responsible for the constant
but subtle reshaping of bone. [link] reviews the bone cells, their functions,
and locations.
Bone Cells
Cell type Function
Osteogenic Develop into
cells osteoblasts
Osteoblasts Bone formation
Maintain mineral
Osteocytes concentration of
matrix
Osteoclasts Bone resorption
Compact and Spongy Bone
Location
Deep layers of the
periosteum and the marrow
Growing portions of bone,
including periosteum and
endosteum
Entrapped in matrix
Bone surfaces and at sites of
old, injured, or unneeded
bone
The differences between compact and spongy bone are best explored via
their histology. Most bones contain compact and spongy osseous tissue, but
their distribution and concentration vary based on the bone’s overall
function. Compact bone is dense so that it can withstand compressive
forces, while spongy (cancellous) bone has open spaces and supports shifts
in weight distribution.
Compact Bone
Compact bone is the denser, stronger of the two types of bone tissue ([Link])
and it provides support and protection.
Diagram of Compact Bone
(a) This cross-sectional view of compact bone shows the basic
structural unit, the osteon. (b) In this micrograph of the osteon, you can
clearly see the concentric lamellae and central canals. LM x 40.
(Micrograph provided by the Regents of University of Michigan
Medical School © 2012)
Compact bone Spongy bone
Medullary cavity
Periosteum
Concentric lamellae
Osteon
Lymphatic vessel
Circumferential
lamellae Nerve
Periosteal artery Blood vessels
Periosteal vein rabeéulae
Periosteum:
Outer fibrous layer
Inner osteogenic layer
Interstitial lamellae Medullar cavity
Perforating canal
Spongy bone
Central canal
Blood vessels
Lymphatic vessel
Nerve
Compact bone
The microscopic structural unit of compact bone is called an osteon, or
Haversian system. Each osteon is composed of concentric rings of calcified
matrix called lamellae (singular = lamella). Running down the center of
each osteon is the central canal, or Haversian canal, which contains blood
vessels, nerves, and lymphatic vessels.
The osteocytes are located inside spaces called lacunae (singular = lacuna),
found at the borders of adjacent lamellae. As described earlier, canaliculi
connect with the canaliculi of other lacunae and eventually with the central
canal. This system allows nutrients to be transported to the osteocytes and
wastes to be removed from them.
Spongy (Cancellous) Bone
Like compact bone, spongy bone, also known as cancellous bone, contains
osteocytes housed in lacunae, but they are not arranged in concentric
circles. Instead, the lacunae and osteocytes are found in a lattice-like
network of matrix spikes called trabeculae (singular = trabecula) ([link]).
The trabeculae may appear to be a random network, but each trabecula
forms along lines of stress to provide strength to the bone. The spaces of the
trabeculated network provide balance to the dense and heavy compact bone
by making bones lighter so that muscles can move them more easily. In
addition, the spaces in some spongy bones contain red marrow, protected by
the trabeculae, where hematopoiesis occurs.
Diagram of Spongy Bone
Spongy bone
Lacuna
Osteocyte
Osteoclast
Osteoblasts aligned
along trabeculae of
new bone
Canaliculi Endosteum _Lamellae .
openings SS Srk Canaliculi
on surface sa.)
Lamellae
Spongy bone is composed of trabeculae that contain
the osteocytes. Red marrow fills the spaces in some
bones.
Note:
Aging and the...
Skeletal System: Paget’s Disease
Paget’s disease usually occurs in adults over age 40. It is a disorder of the
bone remodeling process that begins with overactive osteoclasts. This
means more bone is resorbed than is laid down. The osteoblasts try to
compensate but the new bone they lay down is weak and brittle and
therefore prone to fracture.
While some people with Paget’s disease have no symptoms, others
experience pain, bone fractures, and bone deformities ([link]). Bones of the
pelvis, skull, spine, and legs are the most commonly affected. When
occurring in the skull, Paget’s disease can cause headaches and hearing
loss.
Paget's Disease
Normal Paget’s disease
Normal leg bones are relatively
straight, but those affected by
Paget’s disease are porous and
curved.
What causes the osteoclasts to become overactive? The answer is still
unknown, but hereditary factors seem to play a role. Some scientists
believe Paget’s disease is due to an as-yet-unidentified virus.
Paget’s disease is diagnosed via imaging studies and lab tests. X-rays may
show bone deformities or areas of bone resorption. Bone scans are also
useful. In these studies, a dye containing a radioactive ion is injected into
the body. Areas of bone resorption have an affinity for the ion, so they will
light up on the scan if the ions are absorbed. In addition, blood levels of an
enzyme called alkaline phosphatase are typically elevated in people with
Paget’s disease.
Bisphosphonates, drugs that decrease the activity of osteoclasts, are often
used in the treatment of Paget’s disease. However, in a small percentage of
cases, bisphosphonates themselves have been linked to an increased risk of
fractures because the old bone that is left after bisphosphonates are
administered becomes worn out and brittle. Still, most doctors feel that the
benefits of bisphosphonates more than outweigh the risk; the medical
professional has to weigh the benefits and risks on a case-by-case basis.
Bisphosphonate treatment can reduce the overall risk of deformities or
fractures, which in turn reduces the risk of surgical repair and its associated
risks and complications.
Blood and Nerve Supply
The spongy bone and medullary cavity receive nourishment from arteries
that pass through the compact bone. The arteries enter through the nutrient
foramen (plural = foramina), small openings in the diaphysis ({link]). The
osteocytes in spongy bone are nourished by blood vessels of the periosteum
that penetrate spongy bone and blood that circulates in the marrow cavities.
As the blood passes through the marrow cavities, it is collected by veins,
which then pass out of the bone through the foramina.
In addition to the blood vessels, nerves follow the same paths into the bone
where they tend to concentrate in the more metabolically active regions of
the bone. The nerves sense pain, and it appears the nerves also play roles in
regulating blood supplies and in bone growth, hence their concentrations in
metabolically active sites of the bone.
Diagram of Blood and Nerve Supply to Bone
Articular
Epiphyseal cartilage
artery
and vein Metaphyseal
artery
and vein
Periosteum
Compact
bone
Nutrient
artery
and vein
Nutrient
foramen Medullary cavity
Metaphyseal
artery and vein
Metaphysis —|| “4
Epiphyseal
line
Blood vessels and nerves
enter the bone through the
nutrient foramen.
Chapter Review
A hollow medullary cavity filled with yellow marrow runs the length of the
diaphysis of a long bone. The walls of the diaphysis are compact bone. The
epiphyses, which are wider sections at each end of a long bone, are filled
with spongy bone and red marrow. The epiphyseal plate, a layer of hyaline
cartilage, is replaced by osseous tissue as the organ grows in length.
Bone matrix consists of collagen fibers and organic ground substance,
primarily hydroxyapatite formed from calcium salts. Osteogenic cells
develop into osteoblasts. Osteoblasts are cells that make new bone. They
become osteocytes, the cells of mature bone, when they get trapped in the
matrix. Osteoclasts engage in bone resorption. Compact bone is dense and
composed of osteons, while spongy bone is less dense and made up of
trabeculae. Blood vessels and nerves enter the bone through the nutrient
foramina to nourish and innervate bones.
Review Questions
Exercise:
Problem:
Which of the following occurs in the spongy bone of the epiphysis?
a. bone growth
b. bone remodeling
c. hematopoiesis
d. shock absorption
Solution:
C
Exercise:
Problem: Which of the following are incapable of undergoing mitosis?
a. osteoblasts and osteoclasts
b. osteocytes and osteoclasts
c. osteoblasts and osteocytes
d. osteogenic cells and osteoclasts
Solution:
C
Exercise:
Problem: Which cells do not originate from osteogenic cells?
a. osteoblasts
b. osteoclasts
c. osteocytes
d. osteoprogenitor cells
Solution:
D
Exercise:
Problem:
Which of the following are found in compact bone and cancellous
bone?
a. Haversian systems
b. Haversian canals
c. lamellae
d. lacunae
Solution:
C
Exercise:
Problem:
Which of the following are only found in spongy bone? Hint: See
figure 15.9.
a. canaliculi
b. Volkmann’s canals
c. trabeculae
d. calcium salts
Solution:
C
Exercise:
Problem:
The area of a bone where the nutrient foramen passes forms what kind
of bone marking?
a. a hole
b. a facet
c. a canal
d. a fissure
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
In what ways is the structural makeup of compact and spongy bone
well suited to their respective functions?
Solution:
The densely packed concentric rings of matrix in compact bone are
ideal for resisting compressive forces, which is the function of
compact bone. The open spaces of the trabeculated network of spongy
bone allow spongy bone to support shifts in weight distribution, which
is the function of spongy bone.
Glossary
canaliculi
(singular = canaliculus) channels within the bone matrix that house
one of an osteocyte’s many cytoplasmic extensions that it uses to
communicate and receive nutrients
central canal
longitudinal channel in the center of each osteon; contains blood
vessels, nerves, and lymphatic vessels; also known as the Haversian
canal
compact bone
dense osseous tissue that can withstand compressive forces
diaphysis
tubular shaft that runs between the proximal and distal ends of a long
bone
diploé
layer of spongy bone, that is sandwiched between two the layers of
compact bone found in flat bones
epiphyseal plate
(also, growth plate) sheet of hyaline cartilage in the metaphysis of an
immature bone; replaced by bone tissue as the organ grows in length
epiphysis
wide section at each end of a long bone; filled with spongy bone and
red marrow
hole
opening or depression in a bone
lacunae
(singular = lacuna) spaces in a bone that house an osteocyte
medullary cavity
hollow region of the diaphysis; filled with yellow marrow
nutrient foramen
small opening in the middle of the external surface of the diaphysis,
through which an artery enters the bone to provide nourishment
osteoblast
cell responsible for forming new bone
osteoclast
cell responsible for resorbing bone
osteocyte
primary cell in mature bone; responsible for maintaining the matrix
osteogenic cell
undifferentiated cell with high mitotic activity; the only bone cells that
divide; they differentiate and develop into osteoblasts
osteon
(also, Haversian system) basic structural unit of compact bone; made
of concentric layers of calcified matrix
spongy bone
(also, cancellous bone) trabeculated osseous tissue that supports shifts
in weight distribution
trabeculae
(singular = trabecula) spikes or sections of the lattice-like matrix in
spongy bone
Bone Formation and Development
By the end of this section, you will be able to:
e Explain the function of cartilage
e List the steps of endochondral ossification
e Explain the growth activity at the epiphyseal plate
¢ Compare and contrast the processes of modeling and remodeling
In the early stages of embryonic development, the embryo’s skeleton
consists of fibrous membranes and hyaline cartilage. By the sixth or seventh
week of embryonic life, the actual process of bone development,
ossification, begins.
Cartilage Templates
Bone is a replacement tissue; that is, it uses a model tissue on which to lay
down its mineral matrix. For skeletal development, the most common
template is cartilage. During fetal development, a framework is laid down
that determines where bones will form. Unlike most connective tissues,
cartilage is avascular, meaning that it has no blood vessels supplying
nutrients and removing metabolic wastes. All of these functions are carried
on by diffusion through the matrix. This is why damaged cartilage does not
repair itself as readily as most tissues do.
Throughout fetal development and into childhood growth and development,
bone forms on the cartilaginous matrix. By the time a fetus is born, most of
the cartilage has been replaced with bone. Some additional cartilage will be
replaced throughout childhood, and some cartilage remains in the adult
skeleton.
Endochondral Ossification
In endochondral ossification, bone develops by replacing hyaline
cartilage. Cartilage does not become bone. Instead, cartilage serves as a
template to be completely replaced by new bone.
In a long bone, for example, at about 6 to 8 weeks after conception, some of
the mesenchymal cells differentiate into chondrocytes (cartilage cells) that
form the cartilaginous skeletal precursor of the bones.
Endochondral Ossification
Perichondrium
Primary
ossification
center
Hyaline
cartilage
————
Calcified
matrix
Uncalcified
matrix Uncalcified
matrix
(a) (b) Calcified
matrix
(c) Periosteum
(covers compact
bone) cavity
Artery and vein
(provide nutrients
to bone)
(d)
Secondary
ossification
center
Articular cartilage
sa Artery
, Uncalcified
matrix
Artery and vein
(provide nutrients
to bone)
Endochondral ossification follows five steps. (a)
Mesenchymal cells differentiate into chondrocytes. (b)
The cartilage model of the future bony skeleton. (c)
Capillaries penetrate cartilage. Primary ossification
center develops. (d) Cartilage and chondrocytes continue
to grow at ends of the bone. (e) Secondary ossification
centers develop. (f) Cartilage remains at epiphyseal
(growth) plate and at joint surface as articular cartilage.
As more matrix is produced, the chondrocytes in the center of the
cartilaginous model grow in size. As the matrix calcifies, nutrients can no
longer reach the chondrocytes. This results in their death and the
disintegration of the surrounding cartilage. Blood vessels invade the
resulting spaces, not only enlarging the cavities but also carrying osteogenic
cells with them, many of which will become osteoblasts. These enlarging
Spaces eventually combine to become the medullary cavity.
As the cartilage grows, capillaries penetrate it. This penetration initiates the
transformation of the perichondrium into the bone-producing periosteum.
Here, the osteoblasts form a periosteal collar of compact bone around the
cartilage of the diaphysis. By the second or third month of fetal life, bone
cell development and ossification ramps up and creates the primary
ossification center, a region deep in the periosteal collar where ossification
begins ({link]c).
While these deep changes are occurring, chondrocytes and cartilage
continue to grow at the ends of the bone (the future epiphyses), which
increases the bone’s length at the same time bone is replacing cartilage in
the diaphyses. By the time the fetal skeleton is fully formed, cartilage only
remains at the joint surface as articular cartilage and between the diaphysis
and epiphysis as the epiphyseal plate, the latter of which is responsible for
the longitudinal growth of bones. After birth, this same sequence of events
(matrix mineralization, death of chondrocytes, invasion of blood vessels
from the periosteum, and seeding with osteogenic cells that become
osteoblasts) occurs in the epiphyseal regions, and each of these centers of
activity is referred to as a secondary ossification center ((link]e).
How Bones Grow in Length
The epiphyseal plate is the area of growth in a long bone. It is a layer of
hyaline cartilage where ossification occurs in immature bones. On the
epiphyseal side of the epiphyseal plate, cartilage is formed. On the
diaphyseal side, cartilage is ossified, and the diaphysis grows in length.
Bones continue to grow in length until early adulthood. When the
chondrocytes in the epiphyseal plate cease their proliferation and bone
replaces the cartilage, longitudinal growth stops. All that remains of the
epiphyseal plate is the epiphyseal line ([link]).
Progression from Epiphyseal Plate to Epiphyseal Line
Epiphysis
WZ Metaphysis
Diaphysis
Epiphyseal
Epiphyseal line
plate
(growth
plate)
Ee Metaphysis ——f| >
J __ |} —— Eriphysis ——{__U
Femur
(thighbone)
(a) Growing long bone (b) Mature long bone
As a bone matures, the epiphyseal plate
progresses to an epiphyseal line. (a)
Epiphyseal plates are visible in a growing
bone. (b) Epiphyseal lines are the
remnants of epiphyseal plates in a mature
bone.
How Bones Grow in Diameter
While bones are increasing in length, they are also increasing in diameter;
growth in diameter can continue even after longitudinal growth ceases. This
is called appositional growth. Osteoclasts resorb old bone that lines the
medullary cavity, while osteoblasts, via intramembranous ossification,
produce new bone tissue beneath the periosteum. The erosion of old bone
along the medullary cavity and the deposition of new bone beneath the
periosteum not only increase the diameter of the diaphysis but also increase
the diameter of the medullary cavity. This process is called modeling.
Bone Remodeling
The process in which matrix is resorbed on one surface of a bone and
deposited on another is known as bone modeling. Modeling primarily takes
place during a bone’s growth. However, in adult life, bone undergoes
remodeling, in which resorption of old or damaged bone takes place on the
same surface where osteoblasts lay new bone to replace that which is
resorbed. Injury, exercise, and other activities lead to remodeling. Those
influences are discussed later in the chapter, but even without injury or
exercise, about 5 to 10 percent of the skeleton is remodeled annually just by
destroying old bone and renewing it with fresh bone.
Note:
Diseases of the...
Skeletal System
Osteogenesis imperfecta (OI) is a genetic disease in which bones do not
form properly and therefore are fragile and break easily. It is also called
brittle bone disease. The disease is present from birth and affects a person
throughout life.
The genetic mutation that causes OI affects the body’s production of
collagen, one of the critical components of bone matrix. The severity of the
disease can range from mild to severe. Those with the most severe forms of
the disease sustain many more fractures than those with a mild form.
Frequent and multiple fractures typically lead to bone deformities and short
stature. Bowing of the long bones and curvature of the spine are also
common in people afflicted with OI. Curvature of the spine makes
breathing difficult because the lungs are compressed.
Because collagen is such an important structural protein in many parts of
the body, people with OI may also experience fragile skin, weak muscles,
loose joints, easy bruising, frequent nosebleeds, brittle teeth, blue sclera,
and hearing loss. There is no known cure for OI. Treatment focuses on
helping the person retain as much independence as possible while
minimizing fractures and maximizing mobility. Toward that end, safe
exercises, like swimming, in which the body is less likely to experience
collisions or compressive forces, are recommended. Braces to support legs,
ankles, knees, and wrists are used as needed. Canes, walkers, or
wheelchairs can also help compensate for weaknesses.
When bones do break, casts, splints, or wraps are used. In some cases,
metal rods may be surgically implanted into the long bones of the arms and
legs. Research is currently being conducted on using bisphosphonates to
treat OI. Smoking and being overweight are especially risky in people with
OI, since smoking is known to weaken bones, and extra body weight puts
additional stress on the bones.
Chapter Review
All bone formation is a replacement process. Embryos develop a
cartilaginous skeleton and various membranes. During development, these
are replaced by bone during the ossification process. In endochondral
ossification, bone develops by replacing hyaline cartilage. Activity in the
epiphyseal plate enables bones to grow in length. Modeling allows bones to
grow in diameter. Remodeling occurs as bone is resorbed and replaced by
new bone. Osteogenesis imperfecta is a genetic disease in which collagen
production is altered, resulting in fragile, brittle bones.
Review Questions
Exercise:
Problem: Why is cartilage slow to heal?
a. because it eventually develops into bone
b. because it is semi-solid and flexible
c. because it does not have a blood supply
d. because endochondral ossification replaces all cartilage with bone
Solution:
C
Exercise:
Problem:Bones grow in length due to activity in the
a. epiphyseal plate
b. perichondrium
c. periosteum
d. medullary cavity
Solution:
A
Glossary
endochondral ossification
process in which bone forms by replacing hyaline cartilage
epiphyseal line
completely ossified remnant of the epiphyseal plate
modeling
process, during bone growth, by which bone is resorbed on one surface
of a bone and deposited on another
ossification
(also, osteogenesis) bone formation
remodeling
process by which osteoclasts resorb old or damaged bone at the same
time as and on the same surface where osteoblasts form new bone to
replace that which is resorbed
Muscle Contraction and Locomotion
By the end of this section, you will be able to:
¢ Classify the different types of muscle tissue
e Explain the role of muscles in locomotion
Muscle cells are specialized for contraction. Muscles allow for motions
such as walking, and they also facilitate bodily processes such as respiration
and digestion. The body contains three types of muscle tissue: skeletal
muscle, cardiac muscle, and smooth muscle ((link]).
*
* -v
Skeletal muscle Smooth muscle Cardiac muscle
The body contains three types of muscle tissue: skeletal muscle,
smooth muscle, and cardiac muscle, visualized here using light
microscopy. Skeletal muscle cells are long, striated, and
multinucleate. Smooth muscle cells are short, tapered at each end,
and have only one plump nucleus in each. Cardiac muscle cells
are branched and striated, but short. They also can have more than
one nucleus per cell. (credit: modification of work by NCI, NIH;
scale-bar data from Matt Russell)
Skeletal muscle tissue forms skeletal muscles, which attach to bones or
skin and control locomotion and any movement that can be consciously
controlled. Because it can be controlled by thought, skeletal muscle is also
called voluntary muscle. Skeletal muscles are long and cylindrical in
appearance; when viewed under a microscope, skeletal muscle tissue has a
striped or striated appearance. The striations are caused by the regular
arrangement of contractile proteins (actin and myosin). Actin is a
filamentous contractile protein that interacts with another filamentous
protein called myosin for muscle contraction to occur. Skeletal muscle also
has multiple nuclei present in a single cell.
Smooth muscle tissue occurs in the walls of hollow organs such as the
intestines, stomach, and urinary bladder, and around passages such as the
respiratory tract and blood vessels. Smooth muscle has no striations, is not
under voluntary control, has only one nucleus per cell, is tapered at both
ends, and is called involuntary muscle.
Cardiac muscle tissue is only found in the heart, and cardiac contractions
pump blood throughout the body and maintain blood pressure. Like skeletal
muscle, cardiac muscle is striated, but unlike skeletal muscle, cardiac
muscle cannot be consciously controlled and is called involuntary muscle.
Cardiac muscle cells can have more than one nucleus per cell, are branched,
and are distinguished by the presence of intercalated disks.
Skeletal Muscle Fiber Structure
Each skeletal muscle fiber is a skeletal muscle cell. These cells are
incredibly large, with diameters of up to 100 ym and lengths of up to 30
cm. The plasma membrane of a skeletal muscle fiber is called the
sarcolemma. The sarcolemma is the site of action potential conduction,
which triggers muscle contraction. Within each muscle fiber are myofibrils
—long cylindrical structures that lie parallel to the muscle fiber. Myofibrils
run the entire length of the muscle fiber, and because they are only
approximately 1.2 jam in diameter, hundreds to thousands can be found
inside one muscle fiber. They attach to the sarcolemma at their ends, so that
as myofibrils shorten, the entire muscle cell contracts ([Link]).
Sarcolemma
Sarcoplasm
Striations
Muscle fiber
A skeletal muscle cell is surrounded by a plasma membrane called
the sarcolemma with a cytoplasm called the sarcoplasm. A muscle
fiber is composed of many fibrils, packaged into orderly units.
The striated appearance of skeletal muscle tissue is a result of repeating
bands of the proteins actin and myosin that are present along the length of
myofibrils. The alignment of myofibrils in the cell causes the entire cell to
appear striated or banded.
The Z lines mark the border of units called sarcomeres, which are the
functional units of skeletal muscle. A myofibril is composed of many
sarcomeres running along its length, and as the sarcomeres individually
contract, the myofibrils and muscle cells shorten ((Llink]).
Z line Sarcomere
M line
Thin filament Thick filament
Myofibril
A sarcomere is the region from
one Z line to the next Z line. Many
sarcomeres are present in a
myofibril, resulting in the striation
pattern characteristic of skeletal
muscle.
Myofibrils are composed of smaller structures called myofilaments. There
are two main types of filaments: thick filaments and thin filaments; each
has different compositions and locations. Thick filaments (composed of the
protein myosin) and Thin filaments (composed of the protein actin).
The myosin thick filaments contain two regions designated as the head and
the tail. These regions are important in the process of muscle contraction
and will be discussed in more detail. Two components of the thin filaments
are tropomyosin and troponin. Actin has binding sites for myosin
attachment. Strands of tropomyosin block the binding sites and prevent
actin—myosin interactions when the muscles are at rest. Troponin consists of
three globular subunits. One subunit binds to tropomyosin, one subunit
binds to actin, and one subunit binds Ca?* ions.
Sliding Filament Model of Contraction
For a muscle cell to contract, the sarcomere must shorten. However, thick
and thin filaments—the components of sarcomeres—do not shorten.
Instead, they slide by one another, causing the sarcomere to shorten while
the filaments remain the same length. The sliding filament theory of muscle
contraction was developed to fit the differences observed in the named
bands on the sarcomere at different degrees of muscle contraction and
relaxation. The mechanism of contraction is the binding of myosin to actin,
forming cross-bridges that generate filament movement ([link]).
(a) Thin filament Thick filaments
(actin) (myosin)
Z line M line Z line
aaa naan naan aaaaall Sea ceemeeeeeeeeeceeeeal
Set: Doxa cee eeeeeeeeeeeeeeeel
EEE Dc cae cee eeceeeeeeeeeeeel
SEER SE EEEMEEEEL AIEEE ea
Stteeeteieeettttttettts: >>>
Stitt >>> >
SESE ESSE ESSERE
(b)
| a |
| band A band | band
When (a) a sarcomere (b) contracts, the Z lines move closer
together and the I band gets smaller. The A band stays the same
width and, at full contraction, the thin filaments overlap.
When a sarcomere shortens, some regions shorten whereas others stay the
same length. A sarcomere is defined as the distance between two
consecutive Z lines; when a muscle contracts, the distance between the Z
lines is reduced. Thin filaments are pulled by the thick filaments toward the
center of the sarcomere until the Z lines approach the thick filaments. The
zone of overlap, in which thin filaments and thick filaments occupy the
same area, increases as the thin filaments move inward.
ATP and Muscle Contraction
The motion of muscle shortening occurs as myosin heads bind to actin and
pull the actin inwards. This action requires energy, which is provided by
ATP. Myosin binds to actin at a binding site on the actin protein. Myosin
has another binding site for ATP at which enzymatic activity hydrolyzes
ATP to ADP, releasing an inorganic phosphate molecule and energy.
ATP binding causes myosin to release actin, allowing actin and myosin to
detach from each other. After this happens, the newly bound ATP is
converted to ADP and inorganic phosphate, P;. The enzyme at the binding
site on myosin is called ATPase. The energy released during ATP hydrolysis
changes the angle of the myosin head into a “cocked” position. The myosin
head is then in a position for further movement, possessing potential energy,
but ADP and P; are still attached. If actin binding sites are covered and
unavailable, the myosin will remain in the high energy configuration with
ATP hydrolyzed, but still attached.
If the actin binding sites are uncovered, a cross-bridge will form; that is, the
myosin head spans the distance between the actin and myosin molecules. P;
is then released, allowing myosin to expend the stored energy as a
conformational change. The myosin head moves toward the M line, pulling
the actin along with it. As the actin is pulled, the filaments move
approximately 10 nm toward the M line. This movement is called the power
stroke, as it is the step at which force is produced. As the actin is pulled
toward the M line, the sarcomere shortens and the muscle contracts.
When the myosin head is “cocked,” it contains energy and is in a high-
energy configuration. This energy is expended as the myosin head moves
through the power stroke; at the end of the power stroke, the myosin head is
in a low-energy position. After the power stroke, ADP is released; however,
the cross-bridge formed is still in place, and actin and myosin are bound
together. ATP can then attach to myosin, which allows the cross-bridge
cycle to start again and further muscle contraction can occur ([link]).
Note:
Art Connection
Tropomyosin
y GY.
The active site on actin is
exposed as Ca?* binds 2 y
~ Active site
troponin. :
Myosin head
e. myosin head forms
a cross-bridge with actin.
During the power stroke,
@® the myosin head bends,
and ADP and phosphate
are released.
Anew molecule of ATP
@ attaches to the myosin
head, causing the
cross-bridge to detach.
ATP hydrolyzes to ADP
6 and phosphate, which
returns the myosin to the
“cocked” position.
The cross-bridge muscle
contraction cycle, which is
triggered by Ca** binding to the
actin active site, is shown. With
each contraction cycle, actin
moves relative to myosin.
Regulatory Proteins
When a muscle is in a resting state, actin and myosin are separated. To keep
actin from binding to the active site on myosin, regulatory proteins block
the molecular binding sites. Tropomyosin blocks myosin binding sites on
actin molecules, preventing cross-bridge formation and preventing
contraction in a muscle without nervous input. Troponin binds to
tropomyosin and helps to position it on the actin molecule; it also binds
calcium ions.
To enable a muscle contraction, tropomyosin must change conformation,
uncovering the myosin-binding site on an actin molecule and allowing
cross-bridge formation. This can only happen in the presence of calcium,
which is kept at extremely low concentrations in the sarcoplasm. If present,
calcium ions bind to troponin, causing conformational changes in troponin
that allow tropomyosin to move away from the myosin binding sites on
actin. Once the tropomyosin is removed, a cross-bridge can form between
actin and myosin, triggering contraction. Cross-bridge cycling continues
until Ca** ions and ATP are no longer available and tropomyosin again
covers the binding sites on actin.
Excitation—Contraction Coupling
Excitation—contraction coupling is the link (transduction) between the
action potential generated in the sarcolemma and the start of a muscle
contraction. The trigger for calcium release from the sarcoplasmic
reticulum into the sarcoplasm is a neural signal. Each skeletal muscle fiber
is controlled by a motor neuron, which conducts signals from the brain or
spinal cord to the muscle. The area of the sarcolemma on the muscle fiber
that interacts with the neuron is called the motor end plate. The end of the
neuron’s axon is called the synaptic terminal, and it does not actually
contact the motor end plate. A small space called the synaptic cleft
separates the synaptic terminal from the motor end plate. Electrical signals
travel along the neuron’s axon, which branches through the muscle and
connects to individual muscle fibers at a neuromuscular junction.
The ability of cells to communicate electrically requires that the cells
expend energy to create an electrical gradient across their cell membranes.
This charge gradient is carried by ions, which are differentially distributed
across the membrane. Each ion exerts an electrical influence and a
concentration influence. Just as milk will eventually mix with coffee
without the need to stir, ions also distribute themselves evenly, if they are
permitted to do so. In this case, they are not permitted to return to an evenly
mixed state.
Transport proteins called sodium-potassium pumps use cellular energy
(ATP) to pump two K" ions inside the cell and three Na” ions outside at the
same time. Therefore, a concentration gradient for both ions exists across
the plasma membrane. This alone accumulates a small electrical charge, but
a big concentration gradient. There is lots of K* in the cell and lots of Na*
outside the cell. Potassium is able to leave the cell through K* channels that
are open 90% of the time, and it does. However, Na™ channels are rarely
open, so Na” remains outside the cell. When K" leaves the cell, obeying its
concentration gradient, that effectively leaves a negative charge behind. So
at rest, there is a large concentration gradient for Na‘ to enter the cell, and
there is an accumulation of negative charges left behind in the cell. This is
the resting membrane potential. Potential in this context means a separation
of electrical charge that is capable of doing work. It is measured in volts,
just like a battery. However, the transmembrane potential is considerably
smaller (0.07 V); therefore, the small value is expressed as millivolts (mV)
or 70 mV. Because the inside of a cell is negative compared with the
outside, a minus sign signifies the excess of negative charges inside the cell,
—70 mV.
If an event changes the permeability of the membrane to Na’ ions, they will
enter the cell. That will change the voltage. This is an electrical event,
called an action potential, that can be used as a cellular signal.
Communication occurs between nerves and muscles through
neurotransmitters. Neuron action potentials cause the release of
neurotransmitters from the synaptic terminal into the synaptic cleft, where
they can then diffuse across the synaptic cleft and bind to a receptor
molecule on the motor end plate. The motor end plate possesses junctional
folds—folds in the sarcolemma that create a large surface area for the
neurotransmitter to bind to receptors. The receptors are actually sodium
channels that open to allow the passage of Na’ into the cell when they
receive neurotransmitter signal.
Acetylcholine (ACh) is a neurotransmitter released by motor neurons that
binds to receptors in the motor end plate. Neurotransmitter release occurs
when an action potential travels down the motor neuron’s axon, resulting in
altered permeability of the synaptic terminal membrane and an influx of
calcium. The Ca?* ions allow synaptic vesicles to move to and bind with the
presynaptic membrane (on the neuron), and release neurotransmitter from
the vesicles into the synaptic cleft. Once released by the synaptic terminal,
ACh diffuses across the synaptic cleft to the motor end plate, where it binds
with ACh receptors. As a neurotransmitter binds, these ion channels open,
and Na’ ions cross the membrane into the muscle cell. This reduces the
voltage difference between the inside and outside of the cell, which is called
depolarization. As ACh binds at the motor end plate, this depolarization is
called an end-plate potential. The depolarization then spreads along the
sarcolemma, creating an action potential as sodium channels adjacent to the
initial depolarization site sense the change in voltage and open. The action
potential moves across the entire cell, creating a wave of depolarization.
ACh is broken down by the enzyme acetylcholinesterase (AChE) into
acetyl and choline. AChE resides in the synaptic cleft, breaking down ACh
so that it does not remain bound to ACh receptors, which would cause
unwanted extended muscle contraction ([link]).
Note:
Art Connection
(a)
Axon terminal
Synaptic vesicles
: Sarcolemma
Acetylcholine (muscle cell
receptor plasma
Synaptic cleft membrane)
Acetylcholine Acetylcholine
Acetylcholinesterase
Sarcoplasmic
reticulum
(muscle cell
endoplasmic
reticulum)
1. Acetylcholine released from the axon 5. Acetylcholinesterase removes acetylcholine
terminal binds to receptors on the from the synaptic cleft.
sarcolemma. 6. Ca?* is transported back into the
2. An action potential is generated and travels sarcoplasmic reticulum.
down the T tubule. 7. Tropomyosin binds active sites on actin
3. Ca** is released from the sarcoplasmic causing the cross-bridge to detach.
reticulum in response to the change in
voltage.
4. Ca?* binds troponin; Cross-bridges form
between actin and myosin.
This diagram shows excitation-contraction
coupling in a skeletal muscle contraction and
contains details not included in the text. Therefore,
it is essential that you read it carefully and study
each step. The sarcoplasmic reticulum (SR) is a
specialized endoplasmic reticulum found in
muscle cells that stores calcium ions. The action
potential within skeletal muscle travels down the T
(Transverse) tubules and triggers the release of
calcium ions from the SR. The calcium ions bind
with troponin/tropomyosin to uncover the myosin
binding sites so that contraction can occur.
After depolarization, the membrane returns to its resting state. This is called
repolarization, during which voltage-gated sodium channels close.
Potassium channels continue at 90% conductance. Because the plasma
membrane sodium—potassium ATPase always transports ions, the resting
State (negatively charged inside relative to the outside) is restored. The
period immediately following the transmission of an impulse in a nerve or
muscle, in which a neuron or muscle cell regains its ability to transmit
another impulse, is called the refractory period. During the refractory
period, the membrane cannot generate another action potential. . The
refractory period allows the voltage-sensitive ion channels to return to their
resting configurations. The sodium potassium ATPase continually moves
Na’ back out of the cell and K* back into the cell, and the K* leaks out
leaving negative charge behind. Very quickly, the membrane repolarizes, so
that it can again be depolarized.
Control of Muscle Tension
Neural control initiates the formation of actin—myosin cross-bridges,
leading to the sarcomere shortening involved in muscle contraction. These
contractions extend from the muscle fiber through connective tissue to pull
on bones, causing skeletal movement. The pull exerted by a muscle is
called tension, and the amount of force created by this tension can vary.
This enables the same muscles to move very light objects and very heavy
objects. In individual muscle fibers, the amount of tension produced
depends on the cross-sectional area of the muscle fiber and the frequency of
neural stimulation.
The number of cross-bridges formed between actin and myosin determine
the amount of tension that a muscle fiber can produce. Cross-bridges can
only form where thick and thin filaments overlap, allowing myosin to bind
to actin. If more cross-bridges are formed, more myosin will pull on actin,
and more tension will be produced.
The ideal length of a sarcomere during production of maximal tension
occurs when thick and thin filaments overlap to the greatest degree. If a
sarcomere at rest is stretched past an ideal resting length, thick and thin
filaments do not overlap to the greatest degree, and fewer cross-bridges can
form. This results in fewer myosin heads pulling on actin, and less tension
is produced. As a sarcomere is shortened, the zone of overlap is reduced as
the thin filaments reach the H zone, which is composed of myosin tails.
Because it is myosin heads that form cross-bridges, actin will not bind to
myosin in this zone, reducing the tension produced by this myofiber. If the
sarcomere is shortened even more, thin filaments begin to overlap with each
other—reducing cross-bridge formation even further, and producing even
less tension. Conversely, if the sarcomere is stretched to the point at which
thick and thin filaments do not overlap at all, no cross-bridges are formed
and no tension is produced. This amount of stretching does not usually
occur because accessory proteins, internal sensory nerves, and connective
tissue oppose extreme stretching.
The primary variable determining force production is the number of
myofibers within the muscle that receive an action potential from the
neuron that controls that fiber. When using the biceps to pick up a pencil,
the motor cortex of the brain only signals a few neurons of the biceps, and
only a few myofibers respond. In vertebrates, each myofiber responds fully
if stimulated. When picking up a piano, the motor cortex signals a majority
of the neurons in the biceps and most myofibers participate. This is close to
the maximum force the muscle can produce. As mentioned above,
increasing the frequency of action potentials (the number of signals per
second) can increase the force a bit more, because the tropomyosin is
flooded with calcium. Even at maximum voluntary contraction, not all
motor units are active simultaneously; at any given moment, some of them
are at rest on a rotational basis.
Section Summary
The body contains three types of muscle tissue: skeletal muscle, cardiac
muscle, and smooth muscle. Skeletal muscle tissue is composed of
sarcomeres, the functional units of muscle tissue. Muscle contraction occurs
when sarcomeres shorten, as thick and thin filaments slide past each other,
which is called the sliding filament model of muscle contraction. ATP
provides the energy for cross-bridge formation and filament sliding.
Regulatory proteins, such as troponin and tropomyosin, control cross-bridge
formation. Excitation—contraction coupling transduces the electrical signal
of the neuron, via acetylcholine, to an electrical signal on the muscle
membrane, which initiates force production. The number of muscle fibers
contracting determines how much force the whole muscle produces.
Art Connections
Exercise:
Problem:
[link] Which of the following statements about muscle contraction is
true?
a. The power stroke occurs when ATP is hydrolyzed to ADP and
phosphate.
b. The power stroke occurs when ADP and phosphate dissociate
from the myosin head.
c. The power stroke occurs when ADP and phosphate dissociate
from the actin active site.
d. The power stroke occurs when Ca?* binds the calcium head.
Solution:
[link] B
Exercise:
Problem:
[link] The deadly nerve gas Sarin irreversibly inhibits
acetycholinesterase. What effect would Sarin have on muscle
contraction?
Solution:
[link] In the presence of Sarin, acetycholine is not removed from the
synapse, resulting in continuous stimulation of the muscle plasma
membrane. At first, muscle activity is intense and uncontrolled, but the
ion gradients dissipate, so electrical signals in the T-tubules are no
longer possible. The result is paralysis, leading to death by
asphyxiation.
Review Questions
Exercise:
Problem:
In relaxed muscle, the myosin-binding site on actin is blocked by
a. titin
b. troponin
c. myoglobin
d. tropomyosin
Solution:
D
Exercise:
Problem:The cell membrane of a muscle fiber is called a
a. myofibril
b. sarcolemma
c. sarcoplasm
d. myofilament
Solution:
B
Exercise:
Problem:
The muscle relaxes if no new nerve signal arrives. However the
neurotransmitter from the previous stimulation is still present in the
synapse. The activity of helps to remove this
neurotransmitter.
a. myosin
b. action potential
c. tropomyosin
d. acetylcholinesterase
Solution:
D
Free Response
Exercise:
Problem:
How would muscle contractions be affected if ATP was completely
depleted in a muscle fiber?
Solution:
Because ATP is required for myosin to release from actin, muscles
would remain rigidly contracted until more ATP was available for the
myosin cross-bridge release. This is why dead vertebrates undergo
rigor mortis.
Glossary
actin
globular contractile protein that interacts with myosin for muscle
contraction
acetylcholinesterase
(AChE) enzyme that breaks down ACh into acetyl and choline
cardiac muscle
tissue muscle tissue found only in the heart; cardiac contractions pump
blood throughout the body and maintain blood pressure
motor end plate
sarcolemma of the muscle fiber that interacts with the neuron
myofibril
long cylindrical structures that lie parallel to the muscle fiber
myofilament
small structures that make up myofibrils
myosin
contractile protein that interacts with actin for muscle contraction
sarcolemma
plasma membrane of a skeletal muscle fiber
sarcomere
functional unit of skeletal muscle
skeletal muscle tissue
forms skeletal muscles, which attach to bones and control locomotion
and any movement that can be consciously controlled
smooth muscle
tissue occurs in the walls of hollow organs such as the intestines,
stomach, and urinary bladder, and around passages such as the
respiratory tract and blood vessels
thick filament
a group of myosin molecules
thin filament
two polymers of actin wound together along with tropomyosin and
troponin
tropomyosin
acts to block myosin binding sites on actin molecules, preventing
cross-bridge formation and preventing contraction until a muscle
receives a neuron signal
troponin
binds to tropomyosin and helps to position it on the actin molecule,
and also binds calcium ions
Introduction to the Nervous System
class="introduction"
An athlete’s
nervous
system is
hard at work
during the
planning
and
execution of
a movement
as precise as
a high jump.
Parts of the
nervous
system are
involved in
determining
how hard to
push off and
when to
turn, as well
as
controlling
the muscles
throughout
the body
that make
this
complicated
movement
possible
without
knocking
the bar
down—all
in just a few
seconds.
(credit:
modificatio
n of work
by Shane T.
McCoy,
U.S. Navy)
=
SAWYS RKiON ©
QWariivg OA
When you’re reading this book, your nervous system is performing several
functions simultaneously. The visual system is processing what is seen on
the page; the motor system controls the turn of the pages (or click of the
mouse); the prefrontal cortex maintains attention. Even fundamental
functions, like breathing and regulation of body temperature, are controlled
by the nervous system. A nervous system is an organism’s control center: it
processes sensory information from outside (and inside) the body and
controls all behaviors—from eating to sleeping to finding a mate.
Neurons and Glial Cells
By the end of this section, you will be able to:
e List and describe the functions of the structural components of a
neuron
e Describe the structure multipolar neurons
e State the general function of glial cells
The nervous system is made up of neurons, specialized cells that can
receive and transmit chemical or electrical signals, and glia, cells that
provide support functions for the neurons by playing an information
processing role that is complementary to neurons. A neuron can be
compared to an electrical wire—it transmits a signal from one place to
another. Glia can be compared to the workers at the electric company who
make sure wires go to the right places, maintain the wires, and take down
wires that are broken. Although glia have been compared to workers, recent
evidence suggests that also usurp some of the signaling functions of
neurons.
There is great diversity in the types of neurons and glia that are present in
different parts of the nervous system. There are four major types of neurons,
and they share several important cellular components.
Neurons
The nervous system of the common laboratory fly, Drosophila
melanogaster, contains around 100,000 neurons, the same number as a
lobster. This number compares to 75 million in the mouse and 300 million
in the octopus. A human brain contains around 86 billion neurons. Despite
these very different numbers, the nervous systems of these animals control
many of the same behaviors—from basic reflexes to more complicated
behaviors like finding food and courting mates. The ability of neurons to
communicate with each other as well as with other types of cells underlies
all of these behaviors.
Most neurons share the same cellular components. But neurons are also
highly specialized—different types of neurons have different sizes and
shapes that relate to their functional roles.
Parts of a Neuron
Like other cells, each neuron has a cell body that contains a nucleus,
smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria,
and other cellular components. Neurons also contain unique structures,
illustrated in [link] for receiving and sending the electrical signals that make
neuronal communication possible. Dendrites are tree-like structures that
extend away from the cell body to receive messages from other neurons at
specialized junctions called synapses. Although some neurons do not have
any dendrites, some types of neurons have multiple dendrites. Dendrites can
have small protrusions called dendritic spines, which further increase
surface area for possible synaptic connections.
Once a signal is received by the dendrite, it then travels passively to the cell
body. The cell body contains a specialized structure, the axon hillock that
integrates signals from multiple synapses and serves as a junction between
the cell body and an axon. An axon is a tube-like structure that propagates
the integrated signal to specialized endings called axon terminals. These
terminals in turn synapse on other neurons, muscle, or target organs.
Chemicals released at axon terminals allow signals to be communicated to
these other cells. Neurons usually have one or two axons, but some neurons,
like amacrine cells in the retina, do not contain any axons. Some axons are
covered with myelin, which acts as an insulator to minimize dissipation of
the electrical signal as it travels down the axon, greatly increasing the speed
on conduction. This insulation is important as the axon from a human motor
neuron can be as long as a meter—from the base of the spine to the toes.
The myelin sheath is not actually part of the neuron. Myelin is produced by
glial cells. Along the axon there are periodic gaps in the myelin sheath.
These gaps are called nodes of Ranvier and are sites where the signal is
“recharged” as it travels along the axon.
It is important to note that a single neuron does not act alone—neuronal
communication depends on the connections that neurons make with one
another (as well as with other cells, like muscle cells). Dendrites from a
single neuron may receive synaptic contact from many other neurons. For
example, dendrites from a Purkinje cell in the cerebellum are thought to
receive contact from as many as 200,000 other neurons.
Note:
Art Connection
Cell body (soma)
Oligodendrocyte
Node of Ranvier
Cell membrane
Dendrite
Myelin sheath
Synapse — ; ‘\ \\-
Neurons contain organelles common to many other cells,
such as a nucleus and mitochondria. They also have more
specialized structures, including dendrites and axons.
Types of Neurons
There are different types of neurons, and the functional role of a given
neuron is intimately dependent on its structure. There is an amazing
diversity of neuron shapes and sizes found in different parts of the nervous
system (and across species), as illustrated by the neurons shown in [Link].
aff an
TERIA
(a) Pyramidal cell of the (b) Purkinje cell of the (c) Olfactory neurons
cerebral cortex cerebellar cortex
There is great diversity in the size and shape of neurons
throughout the nervous system. Examples include (a) a
pyramidal cell from the cerebral cortex, (b) a Purkinje cell
from the cerebellar cortex, and (c) olfactory cells from the
olfactory epithelium and olfactory bulb.
Multipolar neurons are the most common type of neuron. Each multipolar
neuron contains one axon and multiple dendrites. Multipolar neurons can be
found in the central nervous system (brain and spinal cord). An example of
a multipolar neuron is a Purkinje cell in the cerebellum, which has many
branching dendrites but only one axon.
Unipolar neuron Multipolar neuron
Bipolar neuron Pseudounipolar neuron
Neurons are broadly divided into four main types based on the
number and placement of axons: (1) unipolar, (2) bipolar, (3)
multipolar, and (4) pseudounipolar.
Note:
Everyday Connection
Neurogenesis
At one time, scientists believed that people were born with all the neurons
they would ever have. Research performed during the last few decades
indicates that neurogenesis, the birth of new neurons, continues into
adulthood. Neurogenesis was first discovered in songbirds that produce
new neurons while learning songs. For mammals, new neurons also play
an important role in learning: about 1000 new neurons develop in the
hippocampus (a brain structure involved in learning and memory) each
day. While most of the new neurons will die, researchers found that an
increase in the number of surviving new neurons in the hippocampus
correlated with how well rats learned a new task. Interestingly, both
exercise and some antidepressant medications also promote neurogenesis
in the hippocampus. Stress has the opposite effect. While neurogenesis is
quite limited compared to regeneration in other tissues, research in this
area may lead to new treatments for disorders such as Alzheimer’s, stroke,
and epilepsy.
How do scientists identify new neurons? A researcher can inject a
compound called bromodeoxyuridine (BrdU) into the brain of an animal.
While all cells will be exposed to BrdU, BrdU will only be incorporated
into the DNA of newly generated cells that are in S phase. A technique
called immunohistochemistry can be used to attach a fluorescent label to
the incorporated BrdU, and a researcher can use fluorescent microscopy to
visualize the presence of BrdU, and thus new neurons, in brain tissue.
[link] is a micrograph which shows fluorescently labeled neurons in the
hippocampus of a rat.
BrdU/Nestin
Astrocyte
This micrograph shows
fluorescently labeled new
neurons in a rat hippocampus.
Cells that are actively dividing
have bromodoxyuridine (BrdU)
incorporated into their DNA
and are labeled in red. Cells that
express glial fibrillary acidic
protein (GFAP) are labeled in
green. Astrocytes, but not
neurons, express GFAP. Thus,
cells that are labeled both red
and green are actively dividing
astrocytes, whereas cells labeled
red only are actively dividing
neurons. (credit: modification of
work by Dr. Maryam Faiz, et.
al., University of Barcelona;
scale-bar data from Matt
Russell)
Glia
While glia are often thought of as the supporting cast of the nervous system,
the number of glial cells in the brain actually outnumbers the number of
neurons by a factor of ten. Neurons would be unable to function without the
vital roles that are fulfilled by these glial cells. Glia guide developing
neurons to their destinations, buffer ions and chemicals that would
otherwise harm neurons, and provide myelin sheaths around axons.
Scientists have recently discovered that they also play a role in responding
to nerve activity and modulating communication between nerve cells. When
glia do not function properly, the result can be disastrous—most brain
tumors are caused by mutations in glia.
Section Summary
The nervous system is made up of neurons and glia. Neurons are
specialized cells that are capable of sending electrical as well as chemical
signals. Most neurons contain dendrites, which receive these signals, and
axons that send signals to other neurons or tissues. There are four main
types of neurons: unipolar, bipolar, multipolar, and pseudounipolar neurons.
Glia are non-neuronal cells in the nervous system that support neuronal
development and signaling. There are several types of glia that serve
different functions.
Review Questions
Exercise:
Problem:
Neurons contain , which can receive signals from other
neurons.
a. axons
b. mitochondria
c. dendrites
d. Golgi bodies
Solution:
C
Exercise:
Problem: A(n) neuron has one axon and multiple dendrites.
a. unipolar
b. bipolar
c. multipolar
d. pseudounipolar
Solution:
C
Free Response
Exercise:
Problem:
How are neurons similar to other cells? How are they unique?
Solution:
Neurons contain organelles common to all cells, such as a nucleus and
mitochondria. They are unique because they contain dendrites, which
can receive signals from other neurons, and axons that can send these
signals to other cells.
Exercise:
Problem:
Multiple sclerosis causes demyelination of axons in the brain and
spinal cord. Why is this problematic?
Solution:
Myelin provides insulation for signals traveling along axons. Without
myelin, signal transmission can slow down and degrade over time.
This would slow down neuronal communication across the nervous
system and affect all downstream functions.
Glossary
axon
tube-like structure that propagates a signal from a neuron’s cell body to
axon terminals
axon hillock
electrically sensitive structure on the cell body of a neuron that
integrates signals from multiple neuronal connections
axon terminal
structure on the end of an axon that can form a synapse with another
neuron
dendrite
structure that extends away from the cell body to receive messages
from other neurons
glia
(also, glial cells) cells that provide support functions for neurons
myelin
fatty substance produced by glia that insulates axons
neuron
specialized cell that can receive and transmit electrical and chemical
signals
nodes of Ranvier
gaps in the myelin sheath where the signal is recharged
synapse
junction between two neurons where neuronal signals are
communicated
How Neurons Communicate
By the end of this section, you will be able to:
e Describe the basis of the resting membrane potential
e Explain the stages of an action potential and how action potentials are
propagated
e Explain how chemical synapses function
All functions performed by the nervous system—from a simple motor
reflex to more advanced functions like making a memory or a decision—
require neurons to communicate with one another. While humans use words
and body language to communicate, neurons use electrical and chemical
signals. Just like a person in a committee, one neuron usually receives and
synthesizes messages from multiple other neurons before “making the
decision” to send the message on to other neurons.
Nerve Impulse Transmission within a Neuron
For the nervous system to function, neurons must be able to send and
receive signals. These signals are possible because each neuron has a
charged cellular membrane (a voltage difference between the inside and the
outside), and the charge of this membrane can change in response to
neurotransmitter molecules released from other neurons and environmental
stimuli. To understand how neurons communicate, one must first
understand the basis of the baseline or ‘resting’ membrane charge.
Neuronal Charged Membranes
The lipid bilayer membrane that surrounds a neuron is impermeable to
charged molecules or ions. To enter or exit the neuron, ions must pass
through special proteins called ion channels that span the membrane. Ion
channels have different configurations: open, closed, and inactive, as
illustrated in [link]. Some ion channels need to be activated in order to open
and allow ions to pass into or out of the cell. These ion channels are
sensitive to the environment and can change their shape accordingly. Ion
channels that change their structure in response to voltage changes are
called voltage-gated ion channels. Voltage-gated ion channels regulate the
relative concentrations of different ions inside and outside the cell. The
difference in total charge between the inside and outside of the cell is called
the membrane potential.
Voltage-gated Na* Channels
SOCOCK
ee
Cytoplasm
Closed At the resting potential, the Open In response to a nerve impulse, Inactivated For a brief period following
channel is closed. the gate opens and Na* enters the cell. activation, the channel does not open
in response to a new signal.
Voltage-gated ion channels open in response to changes in
membrane voltage. After activation, they become inactivated
for a brief period and will no longer open in response to a
signal.
Resting Membrane Potential
A neuron at rest is negatively charged: the inside of a cell is approximately
70 millivolts more negative than the outside (—70 mV, note that this number
varies by neuron type and by species). This voltage is called the resting
membrane potential; it is caused by differences in the concentrations of ions
inside and outside the cell. If the membrane were equally permeable to all
ions, each type of ion would flow across the membrane and the system
would reach equilibrium. Because ions cannot simply cross the membrane
at will, there are different concentrations of several ions inside and outside
the cell, as shown in [link]. The difference in the number of positively
charged potassium ions (K*) inside and outside the cell dominates the
resting membrane potential ([link]). When the membrane is at rest, K* ions
accumulate inside the cell due to a net movement with the concentration
gradient. The negative resting membrane potential is created and
maintained by increasing the concentration of cations outside the cell (in the
extracellular fluid) relative to inside the cell (in the cytoplasm). The
negative charge within the cell is created by the cell membrane being more
permeable to potassium ion movement than sodium ion movement. In
neurons, potassium ions are maintained at high concentrations within the
cell while sodium ions are maintained at high concentrations outside of the
cell. The cell possesses potassium and sodium leakage channels that allow
the two cations to diffuse down their concentration gradient. However, the
neurons have far more potassium leakage channels than sodium leakage
channels. Therefore, potassium diffuses out of the cell at a much faster rate
than sodium leaks in. Because more cations are leaving the cell than are
entering, this causes the interior of the cell to be negatively charged relative
to the outside of the cell. The actions of the sodium potassium pump help to
maintain the resting potential, once established. Recall that sodium
potassium pumps brings two K” ions into the cell while removing three Na*
ions per ATP consumed. As more cations are expelled from the cell than
taken in, the inside of the cell remains negatively charged relative to the
extracellular fluid. It should be noted that calcium ions (Ca*~) tend to
accumulate outside of the cell because they are repelled by negatively-
charged proteins within the cytoplasm.
Ion Concentration Inside and Outside Neurons
Extracellular Intracellular
concentration concentration Ratio
Ion (mM) (mM) outside/inside
Na* 145 12 12
Ion Concentration Inside and Outside Neurons
Extracellular Intracellular
concentration concentration Ratio
Ion (mM) (mM) outside/inside
K+ 4 155 0.026
Cle 120 4 30
Organic
anions — 100
(A-)
The resting membrane potential is a result of different concentrations inside
and outside the cell.
(a) Resting potential
* 8eeeeeees
© a |
©) Na* /K* transporter (x) (ie) «) ©
At the resting potential, all voltage-gated Na* channels and most voltage-gated K* channels are closed. The Na*/K* transporter
pumps K* ions into the cell and Na* ions out.
(b) Depolarization
Nat /K* transporter
In response to a depolarization, some Na* channels open, allowing Na* ions to enter the cell. The membrane starts to depolarize
(the charge across the membrane lessens). If the threshold of excitation is reached, all the Na* channels open.
(c) Hyperpolarization
Na* channel
800808008
() («) Nat /K* transporter
At the peak action potential, Na* channels close while K* channels open. K* leaves the cell, and the membrane eventually
becomes hyperpolarized.
The (a) resting membrane potential is a result of different
concentrations of Na* and K” ions inside and outside the cell. A nerve
impulse causes Na’ to enter the cell, resulting in (b) depolarization. At
the peak action potential, K* channels open and the cell becomes (c)
hyperpolarized.
Action Potential
A neuron can receive input from other neurons and, if this input is strong
enough, send the signal to downstream neurons. Transmission of a signal
between neurons is generally carried by a chemical called a
neurotransmitter. Transmission of a signal within a neuron (from dendrite to
axon terminal) is carried by a brief reversal of the resting membrane
potential called an action potential. When neurotransmitter molecules bind
to receptors located on a neuron’s dendrites, ion channels open. At
excitatory synapses, this opening allows positive ions to enter the neuron
and results in depolarization of the membrane—a decrease in the
difference in voltage between the inside and outside of the neuron. A
stimulus from a sensory cell or another neuron depolarizes the target neuron
to its threshold potential (-55 mV). Na™ channels in the axon hillock open,
allowing positive ions to enter the cell ({link] and [link]). Once the sodium
channels open, the neuron completely depolarizes to a membrane potential
of about +40 mV. Action potentials are considered an "all-or nothing"
event, in that, once the threshold potential is reached, the neuron always
completely depolarizes. Once depolarization is complete, the cell must now
"reset" its membrane voltage back to the resting potential. To accomplish
this, the Na* channels close and cannot be opened. This begins the neuron's
refractory period, in which it cannot produce another action potential
because its sodium channels will not open. At the same time, voltage-gated
K* channels open, allowing K”* to leave the cell. As K* ions leave the cell,
the membrane potential once again becomes negative. The diffusion of K*
out of the cell actually hyperpolarizes the cell, in that the membrane
potential becomes more negative than the cell's normal resting potential. At
this point, the sodium channels will return to their resting state, meaning
they are ready to open again if the membrane potential again exceeds the
threshold potential. Eventually the extra K” ions diffuse out of the cell
through the potassium leakage channels, bringing the cell from its
hyperpolarized state, back to its resting membrane potential. The sodium-
potassium pumps function to restore the original ion concentrations
associated with the maintenance of the resting membrane potential.
Note:
Art Connection
Peak action potential
+30
Repolarization
Threshold of
excitation
Hyperpolarization
Membrane potential (mV)
Time
The formation of an action
potential can be divided into
five steps: (1) A stimulus from a
sensory cell or another neuron
causes the target cell to
depolarize toward the threshold
potential. (2) If the threshold of
excitation is reached, all Na*
channels open and the
membrane depolarizes. (3) At
the peak action potential, K*
channels open and K* begins to
leave the cell. At the same time,
Na* channels close. (4) The
membrane becomes
hyperpolarized as K* ions
continue to leave the cell. The
hyperpolarized membrane is in
a refractory period and cannot
fire. (5) The K* channels close
and the Na‘/K”™ transporter
restores the resting potential.
SE
Direction of travel of action potential
a. In response to a signal, the
soma end of the axon becomes
depolarized.
b. The depolarization spreads down
the axon. Meanwhile, the first part of
the membrane repolarizes. Because
Na* channels are inactivated and
additional K* channels have opened,
the membrane cannot depolarize again.
c. The action potential continues to
travel down the axon.
The action potential is conducted down the axon as the axon
membrane depolarizes, then repolarizes.
Myelin and the Propagation of the Action Potential
For an action potential to communicate information to another neuron, it
must travel along the axon and reach the axon terminals where it can initiate
neurotransmitter release. The speed of conduction of an action potential
along an axon is influenced by both the diameter of the axon and the axon’s
resistance to ion leakage. Myelin acts as an insulator that prevents current
from leaving the axon; this increases the speed of action potential
conduction. In demyelinating diseases like multiple sclerosis, action
potential conduction slows because ions leak from previously insulated
axon areas. The nodes of Ranvier, illustrated in [link] are gaps in the myelin
sheath along the axon. These unmyelinated spaces are about one
micrometer long and contain voltage gated Na* and K* channels. Flow of
ions through these channels, particularly the Na* channels, regenerates the
action potential over and over again along the axon. This ‘jumping’ of the
action potential from one node to the next is called saltatory conduction. If
nodes of Ranvier were not present along an axon, the action potential would
propagate very slowly since Na* and K* channels would have to
continuously regenerate action potentials at every point along the axon
instead of at specific points. Nodes of Ranvier also save energy for the
neuron since the channels only need to be present at the nodes and not
along the entire axon.
Axon Nodes of Ranvier Myelin sheath
Depolarized Membrane at
membrane resting potential
Nodes of Ranvier are gaps in
myelin coverage along axons.
Nodes contain voltage-gated K*
and Na* channels. Action
potentials travel down the axon by
jumping from one node to the
next.
Synaptic Transmission
The synapse or “gap” is the place where information is transmitted from
one neuron to another. Synapses usually form between axon terminals and
dendritic spines, but this is not universally true. There are also axon-to-
axon, dendrite-to-dendrite, and axon-to-cell body synapses. The neuron
transmitting the signal is called the presynaptic neuron, and the neuron
receiving the signal is called the postsynaptic neuron. Note that these
designations are relative to a particular synapse—most neurons are both
presynaptic and postsynaptic. There are two types of synapses: chemical
and electrical.
Chemical Synapse
When an action potential reaches the axon terminal it depolarizes the
membrane and opens voltage-gated Na* channels. Na* ions enter the cell,
further depolarizing the presynaptic membrane. This depolarization causes
voltage-gated Ca** channels to open. Calcium ions entering the cell initiate
a signaling cascade that causes small membrane-bound vesicles, called
synaptic vesicles, containing neurotransmitter molecules to fuse with the
presynaptic membrane. Synaptic vesicles are shown in [link], which is an
image from a scanning electron microscope.
This pseudocolored image taken
with a scanning electron
microscope shows an axon
terminal that was broken open to
reveal synaptic vesicles (blue and
orange) inside the neuron. (credit:
modification of work by Tina
Carvalho, NIH-NIGMS; scale-bar
data from Matt Russell)
Fusion of a vesicle with the presynaptic membrane causes neurotransmitter
to be released into the synaptic cleft, the extracellular space between the
presynaptic and postsynaptic membranes, as illustrated in [link]. The
neurotransmitter diffuses across the synaptic cleft and binds to receptor
proteins on the postsynaptic membrane.
Presynaptic
neuron
Action potential
arrives at axon
terminal.
Axon terminal
Ca?* entry causes
neurotransmitter-containing
synaptic vesicles to release
their contents by exocytosis.
Synaptic
vesicles
Synaptic cleft Voltage-gated Ca2*
channels open and
Ca?* enters the
axon terminal.
Neurotransmitter diffuses across
the synaptic cleft and binds to
ligand-gated ion channels on the
postsynaptic membrane.
Localized membrane
5 Diffusion away
ymatic from synaps
Binding of neurotransmitter opens ligand-gated © Reuptake by the presynapic neuron, enzymatic degradation,
ion channels, resulting in graded potentials. and diffusion reduce neurotransmitter levels, terminating the
signal.
Communication at chemical synapses requires release of
neurotransmitters. When the presynaptic membrane is
depolarized, voltage-gated Ca** channels open and allow Ca?* to
enter the cell. The calcium entry causes synaptic vesicles to fuse
with the membrane and release neurotransmitter molecules into
the synaptic cleft. The neurotransmitter diffuses across the
synaptic cleft and binds to ligand-gated ion channels in the
postsynaptic membrane, resulting in a localized depolarization or
hyperpolarization of the postsynaptic neuron.
The binding of a specific neurotransmitter causes particular ion channels, in
this case chemically-gated channels, on the postsynaptic membrane to open.
Neurotransmitters can either have excitatory or inhibitory effects on the
postsynaptic membrane, as detailed in [link]. For example, when
acetylcholine is released at the synapse between a nerve and muscle (called
the neuromuscular junction) by a presynaptic neuron, it causes postsynaptic
Na* channels to open. Na’ enters the postsynaptic cell and causes the
postsynaptic membrane to depolarize. This depolarization is called an
excitatory postsynaptic potential (EPSP) and makes the postsynaptic
neuron more likely to fire an action potential. Release of neurotransmitter at
inhibitory synapses causes inhibitory postsynaptic potentials (IPSPs), a
hyperpolarization of the presynaptic membrane. For example, when the
neurotransmitter GABA (gamma-aminobutyric acid) is released from a
presynaptic neuron, it binds to and opens Cl channels. Cl ions enter the
cell and hyperpolarizes the membrane, making the neuron less likely to fire
an action potential.
Once neurotransmission has occurred, the neurotransmitter must be
removed from the synaptic cleft so the postsynaptic membrane can “reset”
and be ready to receive another signal. This can be accomplished in three
ways: the neurotransmitter can diffuse away from the synaptic cleft, it can
be degraded by enzymes in the synaptic cleft, or it can be recycled
(sometimes called reuptake) by the presynaptic neuron. Several drugs act at
this step of neurotransmission. For example, some drugs that are given to
Alzheimer’s patients work by inhibiting acetylcholinesterase, the enzyme
that degrades acetylcholine. This inhibition of the enzyme essentially
increases neurotransmission at synapses that release acetylcholine. Once
released, the acetylcholine stays in the cleft and can continually bind and
unbind to postsynaptic receptors.
Neurotransmitter Function and Location
Neurotransmitter Example Location
CNS
Acetylcholine — and/or
PNS
CNS
Dopamine, serotonin,
Biogenic amine eee and/or
PineP PNS
eer | Glycine, glutamate, aspartate, CNS
gamma aminobutyric acid
CNS
Neuropeptide Substance P, endorphins and/or
PNS
Signal Summation
Sometimes a single EPSP is strong enough to induce an action potential in
the postsynaptic neuron, but often multiple presynaptic inputs must create
EPSPs around the same time for the postsynaptic neuron to be sufficiently
depolarized to fire an action potential. This process is called summation
and occurs at the axon hillock, as illustrated in [link]. Additionally, one
neuron often has inputs from many presynaptic neurons—some excitatory
and some inhibitory—so IPSPs can cancel out EPSPs and vice versa. It is
the net change in postsynaptic membrane voltage that determines whether
the postsynaptic cell has reached its threshold of excitation needed to fire an
action potential. Together, synaptic summation and the threshold for
excitation act as a filter so that random “noise” in the system is not
transmitted as important information.
Axon
hillock Summation of
membrane potentials
at the axon hillock
— EPSPs
——IPSPs
wo
o
Threshold of activation
potential (mV)
|
=
f=)
Membrane
Ar
oO
Time
A single neuron can receive both
excitatory and inhibitory inputs
from multiple neurons, resulting in
local membrane depolarization
(EPSP input) and
hyperpolarization (IPSP input).
All these inputs are added together
at the axon hillock. If the EPSPs
are strong enough to overcome the
IPSPs and reach the threshold of
excitation, the neuron will fire.
Section Summary
Neurons have charged membranes because there are different
concentrations of ions inside and outside of the cell. Voltage-gated ion
channels control the movement of ions into and out of a neuron. When a
neuronal membrane is depolarized to at least the threshold of excitation, an
action potential is fired. The action potential is then propagated along an
axon to the axon terminals. In a chemical synapse, the action potential
causes release of neurotransmitter molecules into the synaptic cleft.
Through binding to postsynaptic receptors, the neurotransmitter can cause
excitatory or inhibitory postsynaptic potentials by depolarizing or
hyperpolarizing, respectively, the postsynaptic membrane.
Art Connections
Exercise:
Problem:
[link] Potassium channel blockers, such as amiodarone and
procainamide, which are used to treat abnormal electrical activity in
the heart, called cardiac dysrhythmia, impede the movement of K+
through voltage-gated K+ channels. Which part of the action potential
would you expect potassium channels to affect?
Solution:
[link] Potassium channel blockers slow the repolarization phase, but
have no effect on depolarization.
Review Questions
Exercise:
Problem:
For a neuron to fire an action potential, its membrane must reach
a. hyperpolarization
b. the threshold of excitation
c. the refractory period
d. inhibitory postsynaptic potential
Solution:
B
Exercise:
Problem:
After an action potential, the opening of additional voltage-gated
channels and the inactivation of sodium channels, cause the
membrane to return to its resting membrane potential.
a. sodium
b. potassium
c. calcium
d. chloride
Solution:
B
Free Response
Exercise:
Problem:
How does myelin aid propagation of an action potential along an
axon? How do the nodes of Ranvier help this process?
Solution:
Myelin prevents the leak of current from the axon. Nodes of Ranvier
allow the action potential to be regenerated at specific points along the
axon. They also save energy for the cell since voltage-gated ion
channels and sodium-potassium transporters are not needed along
myelinated portions of the axon.
Exercise:
Problem: What are the main steps in chemical neurotransmission?
Solution:
An action potential travels along an axon until it depolarizes the
membrane at an axon terminal. Depolarization of the membrane causes
voltage-gated Ca** channels to open and Ca?* to enter the cell. The
intracellular calcium influx causes synaptic vesicles containing
neurotransmitter to fuse with the presynaptic membrane. The
neurotransmitter diffuses across the synaptic cleft and binds to
receptors on the postsynaptic membrane. Depending on the specific
neurotransmitter and postsynaptic receptor, this action can cause
positive (excitatory postsynaptic potential) or negative (inhibitory
postsynaptic potential) ions to enter the cell.
Glossary
action potential
self-propagating momentary change in the electrical potential of a
neuron (or muscle) membrane
depolarization
change in the membrane potential to a less negative value
excitatory postsynaptic potential (EPSP)
depolarization of a postsynaptic membrane caused by neurotransmitter
molecules released from a presynaptic cell
hyperpolarization
change in the membrane potential to a more negative value
inhibitory postsynaptic potential (IPSP)
hyperpolarization of a postsynaptic membrane caused by
neurotransmitter molecules released from a presynaptic cell
membrane potential
difference in electrical potential between the inside and outside of a
cell
refractory period
period after an action potential when it is more difficult or impossible
for an action potential to be fired; caused by inactivation of sodium
channels and activation of additional potassium channels of the
membrane
saltatory conduction
“jumping” of an action potential along an axon from one node of
Ranvier to the next
summation
process of multiple presynaptic inputs creating EPSPs around the same
time for the postsynaptic neuron to be sufficiently depolarized to fire
an action potential
synaptic cleft
space between the presynaptic and postsynaptic membranes
synaptic vesicle
spherical structure that contains a neurotransmitter
threshold of excitation
level of depolarization needed for an action potential to fire
The Central and Peripheral Nervous Systems
By the end of this section, you will be able to:
¢ Describe the basic parts and functions of the central nervous system
e Describe the basic parts and functions of the peripheral nervous system
The Central Nervous System
The central nervous system (CNS) is made up of the brain and spinal cord and is covered with three layers of
protective coverings called meninges (“meninges” is derived from the Greek and means “membranes”) ([link]).
The outermost layer is the dura mater, the middle layer is the web-like arachnoid mater, and the inner layer is the
pia mater, which directly contacts and covers the brain and spinal cord. The space between the arachnoid and pia
maters is filled with cerebrospinal fluid (CSF). The brain floats in CSF, which acts as a cushion and shock
absorber.
Dura mater Arachnoid
mater
Pia mater Cerebral
cortex
The cerebral cortex is covered by three
layers of meninges: the dura,
arachnoid, and pia maters. (credit:
modification of work by Gray's
Anatomy)
The Brain
The brain is the part of the central nervous system that is contained in the cranial cavity of the skull. It includes the
cerebral cortex, limbic system, basal ganglia, thalamus, hypothalamus, cerebellum, brainstem, and retinas. The
outermost part of the brain is a thick piece of nervous system tissue called the cerebral cortex. The cerebral
cortex, limbic system, and basal ganglia make up the two cerebral hemispheres. A thick fiber bundle called the
corpus callosum (corpus = “body”; callosum = “tough”) connects the two hemispheres. Although there are some
brain functions that are localized more to one hemisphere than the other, the functions of the two hemispheres are
largely redundant. In fact, sometimes (very rarely) an entire hemisphere is removed to treat severe epilepsy. While
patients do suffer some deficits following the surgery, they can have surprisingly few problems, especially when
the surgery is performed on children who have very immature nervous systems.
In other surgeries to treat severe epilepsy, the corpus callosum is cut instead of removing an entire hemisphere.
This causes a condition called split-brain, which gives insights into unique functions of the two hemispheres. For
example, when an object is presented to patients’ left visual field, they may be unable to verbally name the object
(and may claim to not have seen an object at all). This is because the visual input from the left visual field crosses
and enters the right hemisphere and cannot then signal to the speech center, which generally is found in the left
side of the brain. Remarkably, if a split-brain patient is asked to pick up a specific object out of a group of objects
with the left hand, the patient will be able to do so but will still be unable to verbally identify it.
Each hemisphere contains regions called lobes that are involved in different functions. Each hemisphere of the
mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes: frontal, parietal,
temporal, and occipital ((link]).
Motor cortex
Somatosensory
cortex
Frontal lobe
Parietal lobe
Olfactory bulb
Temporal lobe ‘\
Brainstem
The human cerebral cortex includes the
frontal, parietal, temporal, and occipital
lobes.
Cerebellum
Spinal cord
|
\
\
The frontal lobe is located at the front of the brain, over the eyes. This lobe contains the olfactory bulb, which
processes smells. The frontal lobe also contains the motor cortex, which is important for planning and
implementing movement. Areas within the motor cortex map to different muscle groups. Neurons in the frontal
lobe also control cognitive functions like maintaining attention, speech, and decision-making. Studies of humans
who have damaged their frontal lobes show that parts of this area are involved in personality, socialization, and
assessing risk. The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in
speech and also reading. Two of the parietal lobe’s main functions are processing somatosensation—touch
sensations like pressure, pain, heat, cold—and processing proprioception—the sense of how parts of the body are
oriented in space. The parietal lobe contains a somatosensory map of the body similar to the motor cortex. The
occipital lobe is located at the back of the brain. It is primarily involved in vision—seeing, recognizing, and
identifying the visual world. The temporal lobe is located at the base of the brain and is primarily involved in
processing and interpreting sounds. It also contains the hippocampus (named from the Greek for “seahorse,”
which it resembles in shape) a structure that processes memory formation. The role of the hippocampus in memory
was partially determined by studying one famous epileptic patient, HM, who had both sides of his hippocampus
removed in an attempt to cure his epilepsy. His seizures went away, but he could no longer form new memories
(although he could remember some facts from before his surgery and could learn new motor tasks).
Interconnected brain areas called the basal ganglia play important roles in movement control and posture. The
basal ganglia also regulate motivation.
The thalamus acts as a gateway to and from the cortex. Sensory and motor impulses go through it from the brain
to the effectors in the peripheral nervous system. It also receives feedback from the cortex. This feedback
mechanism can modulate conscious awareness of sensory and motor inputs depending on the attention and arousal
state of the animal. The thalamus helps regulate consciousness, arousal, and sleep states.
Below the thalamus is the hypothalamus. The hypothalamus controls the endocrine system by sending signals to
the pituitary gland. Among other functions, the hypothalamus is the body’s thermostat—it makes sure the body
temperature is kept at appropriate levels. Neurons within the hypothalamus also regulate circadian rhythms,
sometimes called sleep cycles.
The limbic system is a connected set of structures that regulates emotion, as well as behaviors related to fear and
motivation. It plays a role in memory formation and includes parts of the thalamus and hypothalamus as well as
the hippocampus. One important structure within the limbic system is a temporal lobe structure called the
amygdala. The two amygdala (one on each side) are important both for the sensation of fear and for recognizing
fearful faces.
The cerebellum (cerebellum = “little brain”) sits at the base of the brain on top of the brainstem. The cerebellum
controls balance and aids in coordinating movement and learning new motor tasks. The cerebellum of birds is large
compared to other vertebrates because of the coordination required by flight.
The brainstem connects the rest of the brain with the spinal cord and regulates some of the most important and
basic functions of the nervous system including breathing, swallowing, digestion, sleeping, walking, and sensory
and motor information integration.
Spinal cord
Connecting to the brainstem and extending down the body through the spinal column is the spinal cord. The spinal
cord is a thick bundle of nerve tissue that carries information about the body to the brain and from the brain to the
body. The spinal cord is contained within the meninges and the bones of the vertebral column but is able to
communicate signals to and from the body through its connections with spinal nerves (part of the peripheral
nervous system). A cross-section of the spinal cord looks like a white oval containing a gray butterfly-shape
([link]). Myelinated axons make up the “white matter” and neuron and glia cell bodies (and interneurons) make up
the “gray matter.” Axons and cell bodies in the dorsal spinal cord convey mostly sensory information from the
body to the brain. Axons and cell bodies in the ventral spinal cord primarily transmit signals controlling movement
from the brain to the body.
The spinal cord also controls motor reflexes. These reflexes are quick, unconscious movements—like
automatically removing a hand from a hot object. Reflexes are so fast because they involve local synaptic
connections. For example, the knee reflex that a doctor tests during a routine physical is controlled by a single
synapse between a sensory neuron and a motor neuron. While a reflex may only require the involvement of one or
two synapses, synapses with interneurons in the spinal column transmit information to the brain to convey what
happened (the knee jerked, or the hand was hot).
White matter Gray matter
Dorsal horn
Ventral horn
A cross-section of the spinal cord
shows gray matter (containing cell
bodies and interneurons) and white
matter (containing myelinated axons).
The Peripheral Nervous System
The peripheral nervous system (PNS) is the connection between the central nervous system and the rest of the
body. The PNS can be broken down into the autonomic nervous system, which controls bodily functions without
conscious control, and the sensory-somatic nervous system, which transmits sensory information from the skin,
muscles, and sensory organs to the CNS and sends motor commands from the CNS to the muscles.
Autonomic Nervous System
Parasympathetic Sympathetic
Preganglionic
neuron: soma is
usually in the spine
Preganglionic
neuron: soma is
usually in the brain-
stem or sacral
(toward the bottom)
spinal cord
Neurotransmitter
released from the
preganglionic
synapse:
acetylcholine
Postganglionic
neuron: soma is
usually ina
Postganglionic
neuron: soma is in
ganglion near the a sympathetic
target organ ganglion, located
next to the
spinal cord
pare Neurotransmitter
released from
postganglionic
synapse:
acetylcholine or nitric
oxide
Target organ Target organ
postganglionic
synapse:
norepinephrine
“Rest and digest” “Fight or flight”
response is activated response is activated
In the autonomic nervous system, a preganglionic neuron (originating in the thoraco-lumbar region of the spinal
to a neuron in a ganglion that, in turn, synapses on a target organ. Activation of the sympathetic nervous system c
norepinephrine on the target organ. Activation of the parasympathetic nervous system causes release of acetylc
target organ.
The autonomic nervous system serves as the relay between the CNS and the internal organs. It controls the lungs,
the heart, smooth muscle, and exocrine and endocrine glands and vicera of the abdominal cavity. The autonomic
nervous system controls these organs largely without conscious control; it can continuously monitor the conditions
of these different systems and implement changes as needed. Signaling to the target tissue usually involves two
synapses: a preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn,
synapses on the target organ ((link]). There are two divisions of the autonomic nervous system that often have
opposing effects: the sympathetic nervous system and the parasympathetic nervous system.
The sympathetic nervous system is responsible for the immediate responses an animal makes when it encounters
a dangerous situation. One way to remember this is to think of the “fight-or-flight” response a person feels when
encountering a snake (“snake” and “sympathetic” both begin with “s”). Examples of functions controlled by the
sympathetic nervous system include an accelerated heart rate and inhibited digestion. These functions help prepare
an organism’s body for the physical strain required to escape a potentially dangerous situation or to fend off a
predator.
Parasympathetic Nervous System Sympathetic Nervous System
Constricts pupil J ( Dilates pupil
Stimulates salivation Inhibits salivation
Slows heart rate Increases heart rate
Dilates bronchi
Constricts bronchi
Solar plexus Inhibits digestion
Stimulates digestion
Stimulates the breakdown
of glycogen
Stimulates bile secretion
Stimulates secretion of
adrenaline and noradrenaline
Causes bladder to contract :
Inhibits contraction of bladder
The sympathetic and parasympathetic nervous systems
often have opposing effects on target organs.
While the sympathetic nervous system is activated in stressful situations, the parasympathetic nervous system
allows an animal to “rest and digest.” One way to remember this is to think that during a restful situation like a
picnic, the parasympathetic nervous system is in control (“picnic” and “parasympathetic” both start with “p”).
Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (toward the
bottom) spinal cord ({link]). The parasympathetic nervous system resets organ function after the sympathetic
nervous system is activated including slowing of heart rate, lowered blood pressure, and stimulation of digestion.
The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and motor
neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the
CNS. Motor neurons transmit messages about desired movement from the CNS to the muscles to make them
contract. Without its sensory-somatic nervous system, an animal would be unable to process any information about
its environment (what it sees, feels, hears, and so on) and could not control motor movements. Unlike the
autonomic nervous system, which usually has two synapses between the CNS and the target organ, sensory and
motor neurons usually have only one synapse—one ending of the neuron is at the organ and the other directly
contacts a CNS neuron.
Section Summary
The vertebrate central nervous system contains the brain and the spinal cord, which are covered and protected by
three meninges. The brain contains structurally and functionally defined regions. In mammals, these include the
cortex (which can be broken down into four primary functional lobes: frontal, temporal, occipital, and parietal),
basal ganglia, thalamus, hypothalamus, limbic system, cerebellum, and brainstem—although structures in some of
these designations overlap. While functions may be primarily localized to one structure in the brain, most complex
functions, like language and sleep, involve neurons in multiple brain regions. The spinal cord is the information
superhighway that connects the brain with the rest of the body through its connections with peripheral nerves. It
transmits sensory input and motor output and also controls motor reflexes.
The peripheral nervous system contains both the autonomic and sensory-somatic nervous systems. The autonomic
nervous system provides unconscious control over visceral functions and has two divisions: the sympathetic and
parasympathetic nervous systems. The sympathetic nervous system is activated in stressful situations to prepare
the animal for a “fight-or-flight” response. The parasympathetic nervous system is active during restful periods.
The sensory-somatic nervous system is made of cranial and spinal nerves that transmit sensory information from
skin and muscle to the CNS and motor commands from the CNS to the muscles.
Review Questions
Exercise:
Problem: The part of the brain that is responsible for coordination during movement is the
a. limbic system
b. thalamus
c. cerebellum
d. parietal lobe
Solution:
Cc
Exercise:
Problem: Which part of the nervous system directly controls the digestive system?
a. parasympathetic nervous system
b. central nervous system
c. spinal cord
d. sensory-somatic nervous system
Solution:
A
Free Response
Exercise:
Problem: What are the main functions of the spinal cord?
Solution:
The spinal cord transmits sensory information from the body to the brain and motor commands from the brain
to the body through its connections with peripheral nerves. It also controls motor reflexes.
Exercise:
Problem:
What are the main differences between the sympathetic and parasympathetic branches of the autonomic
nervous system?
Solution:
The sympathetic nervous system prepares the body for “fight or flight,” whereas the parasympathetic nervous
system allows the body to “rest and digest.” Sympathetic neurons release norepinephrine onto target organs;
parasympathetic neurons release acetylcholine. Sympathetic neuron cell bodies are located in sympathetic
ganglia. Parasympathetic neuron cell bodies are located in the brainstem and sacral spinal cord. Activation of
the sympathetic nervous system increases heart rate and blood pressure and decreases digestion and blood
flow to the skin. Activation of the parasympathetic nervous system decreases heart rate and blood pressure
and increases digestion and blood flow to the skin.
Exercise:
Problem: What are the main functions of the sensory-somatic nervous system?
Solution:
The sensory-somatic nervous system transmits sensory information from the skin, muscles, and sensory
organs to the CNS. It also sends motor commands from the CNS to the muscles, causing them to contract.
Glossary
action potential
a momentary change in the electrical potential of a neuron (or muscle) membrane
amygdala
a structure within the limbic system that processes fear
autonomic nervous system
the part of the peripheral nervous system that controls bodily functions
axon
a tube-like structure that propagates a signal from a neuron’s cell body to axon terminals
basal ganglia
an interconnected collections of cells in the brain that are involved in movement and motivation
brainstem
a portion of brain that connects with the spinal cord; controls basic nervous system functions like breathing
and swallowing
central nervous system (CNS)
the nervous system made up of the brain and spinal cord; covered with three layers of protective meninges
cerebellum
the brain structure involved in posture, motor coordination, and learning new motor actions
cerebral cortex
the outermost sheet of brain tissue; involved in many higher-order functions
cerebrospinal fluid (CSF)
a clear liquid that surrounds the brain and fills its ventricles and acts as a shock absorber
corpus callosum
a thick nerve bundle that connects the cerebral hemispheres
dendrite
a structure that extends away from the cell body to receive messages from other neurons
depolarization
a change in the membrane potential to a less negative value
frontal lobe
the part of the cerebral cortex that contains the motor cortex and areas involved in planning, attention, and
language
glia
(also, glial cells) the cells that provide support functions for neurons
hippocampus
the brain structure in the temporal lobe involved in processing memories
hypothalamus
the brain structure that controls hormone release and body homeostasis
limbic system
a connected brain area that processes emotion and motivation
membrane potential
a difference in electrical potential between the inside and outside of a cell
meninges
(singular: meninx) the membranes that cover and protect the central nervous system
myelin sheath
a cellular extension containing a fatty substance produced by glia that surrounds and insulates axons
neuron
a specialized cell that can receive and transmit electrical and chemical signals
occipital lobe
the part of the cerebral cortex that contains visual cortex and processes visual stimuli
parasympathetic nervous system
the division of autonomic nervous system that regulates visceral functions during relaxation
parietal lobe
the part of the cerebral cortex involved in processing touch and the sense of the body in space
peripheral nervous system (PNS)
the nervous system that serves as the connection between the central nervous system and the rest of the body;
consists of the autonomic nervous system and the sensory-somatic nervous system
sensory-somatic nervous system
the system of sensory and motor nerves
spinal cord
a thick fiber bundle that connects the brain with peripheral nerves; transmits sensory and motor information;
contains neurons that control motor reflexes
sympathetic nervous system
the division of autonomic nervous system activated during stressful "fight-or-flight” situations
synapse
a junction between two neurons where neuronal signals are communicated
synaptic cleft
a space between the presynaptic and postsynaptic membranes
temporal lobe
the part of the cerebral cortex that processes auditory input; parts of the temporal lobe are involved in speech,
memory, and emotion processing
thalamus
the brain area that relays sensory information to the cortex
threshold of excitation
the level of depolarization needed for an action potential to fire
Introduction to the Special Senses
class="introduction"
This shark
uses its
senses of
sight and
smell to
hunt, but it
also relies
on its ability
to sense the
electric
fields of
prey, a sense
not present
in most land
animals.
(credit:
modificatio
n of work
by
Hermanus
Backpacker
s Hostel,
South
Africa)
In more advanced animals, the senses are constantly at work, making the
animal aware of stimuli—such as light, or sound, or the presence of a
chemical substance in the external environment—and monitoring
information about the organism’s internal environment. All bilaterally
symmetric animals have a sensory system, and the development of any
species’ sensory system has been driven by natural selection; thus, sensory
systems differ among species according to the demands of their
environments. The shark, unlike most fish predators, is electrosensitive—
that is, sensitive to electrical fields produced by other animals in its
environment. While it is helpful to this underwater predator,
electrosensitivity is a sense not found in most land animals. in this chapter,
information about smell, taste, hearing, and vision is presented.
Taste and Smell
By the end of this section, you will be able to:
e Explain in what way smell and taste stimuli differ from other sensory
stimuli
e Identify the five primary tastes that can be distinguished by humans
e Explain in anatomical terms why a dog’s sense of smell is more acute
than a human’s
Taste, also called gustation, and smell, also called olfaction, are the most
interconnected senses in that both involve molecules of the stimulus
entering the body and bonding to receptors. Smell lets an animal sense the
presence of food other chemicals in the environment that can impact their
survival. Similarly, the sense of taste allows animals to discriminate
between types of foods. While the value of a sense of smell is obvious,
what is the value of a sense of taste? Different tasting foods have different
attributes, both helpful and harmful. For example, sweet-tasting substances
tend to be highly caloric, which could be necessary for survival in lean
times. Bitterness is associated with toxicity, and sourness is associated with
spoiled food. Salty foods are valuable in maintaining homeostasis by
helping the body retain water and by providing ions necessary for cells to
function.
Tastes and Odors
Both taste and odor stimuli are molecules taken in from the environment.
The primary tastes detected by humans are sweet, sour, bitter, salty and
umami. The first four tastes need little explanation. The identification of
umami as a fundamental taste occurred fairly recently— it was identified in
1908 by Japanese scientist Kikunae Ikeda while he worked with seaweed
broth, but it was not widely accepted as a taste that could be physiologically
distinguished until many years later. The taste of umami, also known as
savoriness, is attributable to the taste of the amino acid L-glutamate. In fact,
monosodium glutamate, or MSG, is often used in cooking to enhance the
savory taste of certain foods. What is the adaptive value of being able to
distinguish umami? Savory substances tend to be high in protein.
All odors that we perceive are molecules in the air we breathe. If a
substance does not release molecules into the air from its surface, it has no
smell. And if a human or other animal does not have a receptor that
recognizes a specific molecule, then that molecule has no smell. Humans
have about 350 olfactory receptor subtypes that work in various
combinations to allow us to sense about 10,000 different odors. Compare
that to mice, for example, which have about 1,300 olfactory receptor types,
and therefore probably sense more odors. Both odors and tastes involve
molecules that stimulate specific chemoreceptors. Although humans
commonly distinguish taste as one sense and smell as another, they work
together to create the perception of flavor. A person’s perception of flavor is
reduced if he or she has congested nasal passages.
Reception and Transduction
Odorants (odor molecules) enter the nose and dissolve in the olfactory
epithelium, the mucosa at the back of the nasal cavity (as illustrated in
[link]). The olfactory epithelium is a collection of specialized olfactory
receptors in the back of the nasal cavity that spans an area about 5 cm? in
humans. Recall that sensory cells are neurons. An olfactory receptor,
which is a dendrite of a specialized neuron, responds when it binds certain
molecules inhaled from the environment by sending impulses directly to the
olfactory bulb of the brain. Humans have about 12 million olfactory
receptors, distributed among hundreds of different receptor types that
respond to different odors. Twelve million seems like a large number of
receptors, but compare that to other animals: rabbits have about 100
million, most dogs have about 1 billion, and bloodhounds—dogs selectively
bred for their sense of smell—have about 4 billion. The overall size of the
olfactory epithelium also differs between species, with that of bloodhounds,
for example, being many times larger than that of humans.
Olfactory neurons are bipolar neurons (neurons with two processes from
the cell body). Each neuron has a single dendrite buried in the olfactory
epithelium, and extending from this dendrite are 5 to 20 receptor-laden,
hair-like cilia that trap odorant molecules. The sensory receptors on the cilia
are proteins, and it is the variations in their amino acid chains that make the
receptors sensitive to different odorants. Each olfactory sensory neuron has
only one type of receptor on its cilia, and the receptors are specialized to
detect specific odorants, so the bipolar neurons themselves are specialized.
When an odorant binds with a receptor that recognizes it, the sensory
neuron associated with the receptor is stimulated. Olfactory stimulation is
the only sensory information that directly reaches the cerebral cortex,
whereas other sensations are relayed through the thalamus.
Olfactory
bulb
Olfactory
epithelium
Nerve
endings
Nasal
cavity
Bipolar neuron
(a) (b)
In the human olfactory system, (a) bipolar
olfactory neurons extend from (b) the olfactory
epithelium, where olfactory receptors are
located, to the olfactory bulb. (credit:
modification of work by Patrick J. Lynch,
medical illustrator; C. Carl Jaffe, MD,
cardiologist)
Note:
Evolution Connection
Pheromones
A pheromone is a chemical released by an animal that affects the behavior
or physiology of animals of the same species. Pheromonal signals can have
profound effects on animals that inhale them, but pheromones apparently
are not consciously perceived in the same way as other odors. There are
several different types of pheromones, which are released in urine or as
glandular secretions. Certain pheromones are attractants to potential mates,
others are repellants to potential competitors of the same sex, and still
others play roles in mother-infant attachment. Some pheromones can also
influence the timing of puberty, modify reproductive cycles, and even
prevent embryonic implantation. While the roles of pheromones in many
nonhuman species are important, pheromones have become less important
in human behavior over evolutionary time compared to their importance to
organisms with more limited behavioral repertoires.
The vomeronasal organ (VNO, or Jacobson’s organ) is a tubular, fluid-
filled, olfactory organ present in many vertebrate animals that sits adjacent
to the nasal cavity. It is very sensitive to pheromones and is connected to
the nasal cavity by a duct. When molecules dissolve in the mucosa of the
nasal cavity, they then enter the VNO where the pheromone molecules
among them bind with specialized pheromone receptors. Upon exposure to
pheromones from their own species or others, many animals, including
cats, may display the flehmen response (shown in [link]), a curling of the
upper lip that helps pheromone molecules enter the VNO.
Pheromonal signals are sent, not to the main olfactory bulb, but to a
different neural structure that projects directly to the amygdala (recall that
the amygdala is a brain center important in emotional reactions, such as
fear). The pheromonal signal then continues to areas of the hypothalamus
that are key to reproductive physiology and behavior. While some
scientists assert that the VNO is apparently functionally vestigial in
humans, even though there is a similar structure located near human nasal
cavities, others are researching it as a possible functional system that may,
for example, contribute to synchronization of menstrual cycles in women
living in close proximity.
The flehmen response in this tiger
results in the curling of the upper
lip and helps airborne pheromone
molecules enter the vomeronasal
organ. (credit: modification of
work by "chadh"/Flickr)
Taste
Detecting a taste (gustation) is fairly similar to detecting an odor
(olfaction), given that both taste and smell rely on chemical receptors being
stimulated by certain molecules. The primary organ of taste is the taste bud.
A taste bud is a cluster of gustatory receptors (taste cells) that are located
within the bumps on the tongue called papillae (singular: papilla)
(illustrated in [link]). There are several structurally distinct papillae.
Filiform papillae, which are located across the tongue, are tactile, providing
friction that helps the tongue move substances, and contain no taste cells. In
contrast, fungiform papillae, which are located mainly on the anterior two-
thirds of the tongue, each contain one to eight taste buds and also have
receptors for pressure and temperature. The large circumvallate papillae
contain up to 100 taste buds and form a V near the posterior margin of the
tongue.
a
Circumvallate papillae
Foliate
papillae Fungiform papillae
Filiform papillae
(a) Foliate, circumvallate, and fungiform papillae are
located on different regions of the tongue. (b) Foliate
papillae are prominent protrusions on this light micrograph.
(credit a: modification of work by NCI; scale-bar data from
Matt Russell)
In addition to those two types of chemically and mechanically sensitive
papillae are foliate papillae—tleaf-like papillae located in parallel folds
along the edges and toward the back of the tongue, as seen in the [link]
micrograph. Foliate papillae contain about 1,300 taste buds within their
folds. Finally, there are circumvallate papillae, which are wall-like papillae
in the shape of an inverted “V” at the back of the tongue. Each of these
papillae is surrounded by a groove and contains about 250 taste buds.
Each taste bud’s taste cells are replaced every 10 to 14 days. These are
elongated cells with hair-like processes called microvilli at the tips that
extend into the taste bud pore (illustrate in [link]). Food molecules
(tastants) are dissolved in saliva, and they bind with and stimulate the
receptors on the microvilli. The receptors for tastants are located across the
outer portion and front of the tongue, outside of the middle area where the
filiform papillae are most prominent.
Oral Cavity Microvilli Taste pore
La)
©» Epidermis of tongue
Taste cell
Pores in the tongue allow tastants to enter taste
pores in the tongue. (credit: modification of
work by Vincenzo Rizzo)
In humans, there are five primary tastes, and each taste has only one
corresponding type of receptor. Thus, like olfaction, each receptor is
specific to its stimulus (tastant). Transduction of the five tastes happens
through different mechanisms that reflect the molecular composition of the
tastant. A salty tastant (containing NaCl) provides the sodium ions (Na‘)
that enter the taste neurons and excite them directly. Sour tastants are acids
and belong to the thermoreceptor protein family. Binding of an acid or other
sour-tasting molecule triggers a change in the ion channel and these
increase hydrogen ion (H*) concentrations in the taste neurons, thus
depolarizing them. Sweet, bitter, and umami tastants require a G-protein
coupled receptor. These tastants bind to their respective receptors, thereby
exciting the specialized neurons associated with them.
Both tasting abilities and sense of smell change with age. In humans, the
senses decline dramatically by age 50 and continue to decline. A child may
find a food to be too spicy, whereas an elderly person may find the same
food to be bland and unappetizing.
Section Summary
There are five primary tastes in humans: sweet, sour, bitter, salty, and
umami. Each taste has its own receptor type that responds only to that taste.
Tastants enter the body and are dissolved in saliva. Taste cells are located
within taste buds, which are found on three of the four types of papillae in
the mouth.
Regarding olfaction, there are many thousands of odorants, but humans
detect only about 10,000. Like taste receptors, olfactory receptors are each
responsive to only one odorant. Odorants dissolve in nasal mucosa, where
they excite their corresponding olfactory sensory cells. When these cells
detect an odorant, they send their signals to other structures which
ultimately are conveyed to the CNS.
Review Questions
Exercise:
Problem: Which of the following has the fewest taste receptors?
a. fungiform papillae
b. circumvallate papillae
c. foliate papillae
d. filiform papillae
Solution:
D
Exercise:
Problem:
How many different taste molecules do taste cells each detect?
a. one
b. five
c. ten
d. It depends on the spot on the tongue
Solution:
A
Exercise:
Problem: Salty foods activate the taste cells by ;
a. exciting the taste cell directly
b. causing hydrogen ions to enter the cell
c. causing sodium channels to close
d. binding directly to the receptors
Solution:
A
Free Response
Exercise:
Problem:
From the perspective of the recipient of the signal, in what ways do
pheromones differ from other odorants?
Solution:
Pheromones may not be consciously perceived, and pheromones can
have direct physiological and behavioral effects on their recipients.
Exercise:
Problem:
What might be the effect on an animal of not being able to perceive
taste?
Solution:
The animal might not be able to recognize the differences in food
sources and thus might not be able to discriminate between spoiled
food and safe food or between foods that contain necessary nutrients,
such as proteins, and foods that do not.
Glossary
bipolar neuron
neuron with two processes from the cell body, typically in opposite
directions
glomerulus
in the olfactory bulb, one of the two neural clusters that receives
signals from one type of olfactory receptor
gustation
sense of taste
odorant
airborne molecule that stimulates an olfactory receptor
olfaction
sense of smell
olfactory bulb
neural structure in the vertebrate brain that receives signals from
olfactory receptors
olfactory epithelium
specialized tissue in the nasal cavity where olfactory receptors are
located
olfactory receptor
dendrite of a specialized neuron
papilla
one of the small bump-like projections from the tongue
pheromone
substance released by an animal that can affect the physiology or
behavior of other animals
tastant
food molecule that stimulates gustatory receptors
taste bud
clusters of taste cells
umami
one of the five basic tastes, which is described as “savory” and which
may be largely the taste of L-glutamate
Hearing and Vestibular Sensation
By the end of this section, you will be able to:
e Describe the relationship of amplitude and frequency of a sound wave
to attributes of sound
e Trace the path of sound through the auditory system to the site of
transduction of sound
e Identify the structures of the vestibular system that respond to gravity
Audition, or hearing, is important to humans and to other animals for many
different interactions. It enables an organism to detect and receive
information about danger, such as an approaching predator, and to
participate in communal exchanges like those concerning territories or
mating. On the other hand, although it is physically linked to the auditory
system, the vestibular system is not involved in hearing. Instead, an
animal’s vestibular system detects its own movement, both linear and
angular acceleration and deceleration, and balance.
Sound
Auditory stimuli are sound waves, which are mechanical, pressure waves
that move through a medium, such as air or water. There are no sound
waves in a vacuum since there are no air molecules to move in waves. The
speed of sound waves differs, based on altitude, temperature, and medium,
but at sea level and a temperature of 20° C (68° F), sound waves travel in
the air at about 343 meters per second.
As is true for all waves, there are four main characteristics of a sound wave:
frequency, wavelength, period, and amplitude. Frequency is the number of
waves per unit of time, and in sound is heard as pitch. High-frequency
(=15.000Hz) sounds are higher-pitched (short wavelength) than low-
frequency (long wavelengths; <100Hz) sounds. Frequency is measured in
cycles per second, and for sound, the most commonly used unit is hertz
(Hz), or cycles per second. Most humans can perceive sounds with
frequencies between 30 and 20,000 Hz. Women are typically better at
hearing high frequencies, but everyone’s ability to hear high frequencies
decreases with age. Dogs detect up to about 40,000 Hz; cats, 60,000 Hz;
bats, 100,000 Hz; and dolphins 150,000 Hz, and American shad (Alosa
sapidissima), a fish, can hear 180,000 Hz. Those frequencies above the
human range are called ultrasound.
Amplitude, or the dimension of a wave from peak to trough, in sound is
heard as volume and is illustrated in [link]. The sound waves of louder
sounds have greater amplitude than those of softer sounds. For sound,
volume is measured in decibels (dB). The softest sound that a human can
hear is the zero point. Humans speak normally at 60 decibels.
Wavelength
®
3
2
r-%
E
<x
For sound waves, wavelength
corresponds to pitch. Amplitude of the
wave corresponds to volume. The
sound wave shown with a dashed line
is softer in volume than the sound
wave shown with a solid line. (credit:
NIH)
Reception of Sound
In mammals, sound waves are collected by the external, cartilaginous part
of the ear called the pinna, then travel through the auditory canal and cause
vibration of the thin diaphragm called the tympanum or ear drum, the
innermost part of the outer ear (illustrated in [link]). Interior to the
tympanum is the middle ear. The middle ear holds three small bones called
the ossicles, which transfer energy from the moving tympanum to the inner
ear. The three ossicles are the malleus (also known as the hammer), the
incus (the anvil), and stapes (the stirrup). The aptly named stapes looks
very much like a stirrup. The three ossicles are unique to mammals, and
each plays a role in hearing. The malleus attaches at three points to the
interior surface of the tympanic membrane. The incus attaches the malleus
to the stapes. In humans, the stapes is not long enough to reach the
tympanum. If we did not have the malleus and the incus, then the vibrations
of the tympanum would never reach the inner ear. These bones also
function to collect force and amplify sounds. The ear ossicles are
homologous to bones in a fish mouth: the bones that support gills in fish are
thought to be adapted for use in the vertebrate ear over evolutionary time.
Many animals (frogs, reptiles, and birds, for example) use the stapes of the
middle ear to transmit vibrations to the middle ear.
Stapes
(attached to oval window)
Semicircular
canals
Malleus
Vestibular nerve
Cochlear
nerve
Cochlea
Ear canal :
Tympanic
cavity
Tympanum Eustachian tube
Round window
Sound travels through the outer ear to the middle ear, which
is bounded on its exterior by the tympanic membrane. The
middle ear contains three bones called ossicles that transfer
the sound wave to the oval window, the exterior boundary
of the inner ear. The organ of Corti, which is the organ of
sound transduction, lies inside the cochlea. (credit:
modification of work by Lars Chittka, Axel Brockmann)
Transduction of Sound
Vibrating objects, such as vocal cords, create sound waves or pressure
waves in the air. When these pressure waves reach the ear, the ear
transduces this mechanical stimulus (pressure wave) into a nerve impulse
(electrical signal) that the brain perceives as sound. The pressure waves
strike the tympanum, causing it to vibrate. The mechanical energy from the
moving tympanum transmits the vibrations to the three bones of the middle
ear. The stapes transmits the vibrations to a thin diaphragm called the oval
window, which is the outermost structure of the inner ear. The structures
of the inner ear are found in the labyrinth, a bony, hollow structure that is
the most interior portion of the ear. Here, the energy from the sound wave is
transferred from the stapes through the flexible oval window and to the
fluid of the cochlea. The vibrations of the oval window create pressure
waves in the fluid (perilymph) inside the cochlea. The cochlea is a whorled
structure, like the shell of a snail, and it contains receptors for transduction
of the mechanical wave into an electrical signal (as illustrated in [link]).
Inside the cochlea, the basilar membrane is a mechanical analyzer that
runs the length of the cochlea, curling toward the cochlea’s center.
The mechanical properties of the basilar membrane change along its length,
such that it is thicker, tauter, and narrower at the outside of the whorl
(where the cochlea is largest), and thinner, floppier, and broader toward the
apex, or center, of the whorl (where the cochlea is smallest). Different
regions of the basilar membrane vibrate according to the frequency of the
sound wave conducted through the fluid in the cochlea. For these reasons,
the fluid-filled cochlea detects different wave frequencies (pitches) at
different regions of the membrane. When the sound waves in the cochlear
fluid contact the basilar membrane, it flexes back and forth in a wave-like
fashion. Above the basilar membrane is the tectorial membrane.
Note:
Art Connection
Tectorial
membrane
Apex
4,000 Hz
Basilar Middle canal
membrane
Cochlear
‘ 5, nerve
{A
n
2,000 Hz
Upper canal
a
Oval window >
(sound waves 7 7,000 Hz
enter) nz @ Organ
Round window ~ Lower canal of Corti
(sound waves 20,000Hz — (from the apex membrane
exit) to the round
window)
: Tectorial
Outer hair membrane
cells
Stereocilia
Basilar membrane Cochlear nerve _ Inner hair cell
In the human ear, sound waves cause the stapes to press against the
oval window. Vibrations travel up the fluid-filled interior of the
cochlea. The basilar membrane that lines the cochlea gets
continuously thinner toward the apex of the cochlea. Different
thicknesses of membrane vibrate in response to different frequencies
of sound. Sound waves then exit through the round window. In the
cross section of the cochlea (top right figure), note that in addition to
the upper canal and lower canal, the cochlea also has a middle canal.
The organ of Corti (bottom image) is the site of sound transduction.
Movement of stereocilia on hair cells results in an action potential
that travels along the auditory nerve.
The site of transduction is in the organ of Corti (spiral organ). It is
composed of hair cells held in place above the basilar membrane like
flowers projecting up from soil, with their exposed short, hair-like
stereocilia contacting or embedded in the tectorial membrane above them.
The inner hair cells are the primary auditory receptors and exist in a single
row, numbering approximately 3,500. The stereocilia from inner hair cells
extend into small dimples on the tectorial membrane’s lower surface. The
outer hair cells are arranged in three or four rows. They number
approximately 12,000, and they function to fine tune incoming sound
waves. The longer stereocilia that project from the outer hair cells actually
attach to the tectorial membrane. All of the stereocilia are
mechanoreceptors, and when bent by vibrations they respond by opening a
gated ion channel. As a result, the hair cell membrane is depolarized, and a
signal is transmitted to the chochlear nerve. Intensity (volume) of sound is
determined by how many hair cells at a particular location are stimulated.
The hair cells are arranged on the basilar membrane in an orderly way. The
basilar membrane vibrates in different regions, according to the frequency
of the sound waves impinging on it. Likewise, the hair cells that lay above
it are most sensitive to a specific frequency of sound waves. Hair cells can
respond to a small range of similar frequencies, but they require stimulation
of greater intensity to fire at frequencies outside of their optimal range. The
difference in response frequency between adjacent inner hair cells is about
0.2 percent. Compare that to adjacent piano strings, which are about six
percent different. Place theory, which is the model for how biologists think
pitch detection works in the human ear, states that high frequency sounds
selectively vibrate the basilar membrane of the inner ear near the entrance
port (the oval window). Lower frequencies travel farther along the
membrane before causing appreciable excitation of the membrane. The
basic pitch-determining mechanism is based on the location along the
membrane where the hair cells are stimulated. The place theory is the first
step toward an understanding of pitch perception. Considering the extreme
pitch sensitivity of the human ear, it is thought that there must be some
auditory “sharpening” mechanism to enhance the pitch resolution.
When sound waves produce fluid waves inside the cochlea, the basilar
membrane flexes, bending the stereocilia that attach to the tectorial
membrane. Their bending results in action potentials in the hair cells, and
auditory information travels along the neural endings of the bipolar neurons
of the hair cells (collectively, the auditory nerve) to the brain. When the
hairs bend, they release an excitatory neurotransmitter at a synapse with a
sensory neuron, which then conducts action potentials to the central nervous
system. The cochlear branch of the vestibulocochlear cranial nerve sends
information on hearing. The auditory system is very refined, and there is
some modulation or “sharpening” built in. The brain can send signals back
to the cochlea, resulting in a change of length in the outer hair cells,
sharpening or dampening the hair cells’ response to certain frequencies.
Note:
Link to Learning
openstax COLLEGE
“Ell
Watch an animation of sound entering the outer ear, moving through the
ear structure, stimulating cochlear nerve impulses, and eventually sending
signals to the temporal lobe.
Higher Processing
The inner hair cells are most important for conveying auditory information
to the brain. About 90 percent of the afferent neurons carry information
from inner hair cells, with each hair cell synapsing with 10 or so neurons.
Outer hair cells connect to only 10 percent of the afferent neurons, and each
afferent neuron innervates many hair cells. The afferent, bipolar neurons
that convey auditory information travel from the cochlea to the medulla,
through the pons and midbrain in the brainstem, finally reaching the
primary auditory cortex in the temporal lobe.
Vestibular Information
The stimuli associated with the vestibular system are linear acceleration
(gravity) and angular acceleration and deceleration. Gravity, acceleration,
and deceleration are detected by evaluating the inertia on receptive cells in
the vestibular system. Gravity is detected through head position. Angular
acceleration and deceleration are expressed through turning or tilting of the
head.
The vestibular system has some similarities with the auditory system. It
utilizes hair cells just like the auditory system, but it excites them in
different ways. There are five vestibular receptor organs in the inner ear: the
utricle, the saccule, and three semicircular canals. Together, they make up
what’s known as the vestibular labyrinth that is shown in [link]. The utricle
and saccule respond to acceleration in a straight line, such as gravity. The
roughly 30,000 hair cells in the utricle and 16,000 hair cells in the saccule
lie below a gelatinous layer, with their stereocilia projecting into the gelatin.
Embedded in this gelatin are calcium carbonate crystals—like tiny rocks.
When the head is tilted, the crystals continue to be pulled straight down by
gravity, but the new angle of the head causes the gelatin to shift, thereby
bending the stereocilia. The bending of the stereocilia stimulates the
neurons, and they signal to the brain that the head is tilted, allowing the
maintenance of balance. It is the vestibular branch of the vestibulocochlear
cranial nerve that deals with balance.
Posterior Canal
Superior Canal
Urticle
<
Ne BY Cochlea
7)
a,
Horizontal
Canal
Vestibule
Saccule
The structure of the vestibular
labyrinth is shown. (credit:
modification of work by NIH)
The fluid-filled semicircular canals are tubular loops set at oblique angles.
They are arranged in three spatial planes. The base of each canal has a
swelling that contains a cluster of hair cells. The hairs project into a
gelatinous cap called the cupula and monitor angular acceleration and
deceleration from rotation. They would be stimulated by driving your car
around a corner, turning your head, or falling forward. One canal lies
horizontally, while the other two lie at about 45 degree angles to the
horizontal axis, as illustrated in [link]. When the brain processes input from
all three canals together, it can detect angular acceleration or deceleration in
three dimensions. When the head turns, the fluid in the canals shifts,
thereby bending stereocilia and sending signals to the brain. Upon cessation
accelerating or decelerating—or just moving—the movement of the fluid
within the canals slows or stops. For example, imagine holding a glass of
water. When moving forward, water may splash backwards onto the hand,
and when motion has stopped, water may splash forward onto the fingers.
While in motion, the water settles in the glass and does not splash. Note that
the canals are not sensitive to velocity itself, but to changes in velocity, so
moving forward at 6(0mph with your eyes closed would not give the
sensation of movement, but suddenly accelerating or braking would
stimulate the receptors.
Higher Processing
Hair cells from the utricle, saccule, and semicircular canals also
communicate through bipolar neurons to the cochlear nucleus in the
medulla. Cochlear neurons send descending projections to the spinal cord
and ascending projections to the pons, thalamus, and cerebellum.
Connections to the cerebellum are important for coordinated movements.
There are also projections to the temporal cortex, which account for
feelings of dizziness; projections to autonomic nervous system areas in the
brainstem, which account for motion sickness; and projections to the
primary somatosensory cortex, which monitors subjective measurements of
the external world and self-movement. People with lesions in the vestibular
area of the somatosensory cortex see vertical objects in the world as being
tilted. Finally, the vestibular signals project to certain optic muscles to
coordinate eye and head movements.
Note:
Link to Learning
[=] [=
&
openstax COLLEGE
= “1
i: 7
aL
7
Click through this interactive tutorial to review the parts of the ear and how
they function to process sound.
Section Summary
Audition is important for territory defense, predation, predator defense, and
communal exchanges. The vestibular system, which is not auditory, detects
linear acceleration and angular acceleration and deceleration. Both the
auditory system and vestibular system use hair cells as their receptors.
Auditory stimuli are sound waves. The sound wave energy reaches the outer
ear (pinna, canal, tympanum), and vibrations of the tympanum send the
energy to the middle ear. The middle ear bones shift and the stapes transfers
mechanical energy to the oval window of the fluid-filled inner ear cochlea.
Once in the cochlea, the energy causes the basilar membrane to flex,
thereby bending the stereocilia on receptor hair cells. This activates the
receptors, which send their auditory neural signals to the brain.
The vestibular system has five parts that work together to provide the sense
of direction, thus helping to maintain balance. The utricle and saccule
measure head orientation: their calcium carbonate crystals shift when the
head is tilted, thereby activating hair cells. The semicircular canals work
similarly, such that when the head is turned, the fluid in the canals bends
stereocilia on hair cells. The vestibular hair cells also send signals to the
thalamus and to somatosensory cortex, but also to the cerebellum, the
structure above the brainstem that plays a large role in timing and
coordination of movement.
Art Connections
Exercise:
Problem:
[link] Cochlear implants can restore hearing in people who have a
nonfunctional cochlea. The implant consists of a microphone that
picks up sound. A speech processor selects sounds in the range of
human speech, and a transmitter converts these sounds to electrical
impulses, which are then sent to the auditory nerve. Which of the
following types of hearing loss would not be restored by a cochlear
implant?
a. Hearing loss resulting from absence or loss of hair cells in the
organ of Corti.
b. Hearing loss resulting from an abnormal auditory nerve.
c. Hearing loss resulting from fracture of the cochlea.
d. Hearing loss resulting from damage to bones of the middle ear.
Solution:
[link] B
Review Questions
Exercise:
Problem:
In sound, pitch is measured in , and volume is measured in
a. nanometers (nm); decibels (dB)
b. decibels (dB); nanometers (nm)
c. decibels (dB); hertz (Hz)
d. hertz (Hz); decibels (dB)
Solution:
D
Exercise:
Problem: Auditory hair cells are indirectly anchored to the
a. basilar membrane
b. oval window
c. tectorial membrane
d. ossicles
Solution:
A
Exercise:
Problem:
Which of the following are found both in the auditory system and the
vestibular system?
a. basilar membrane
b. hair cells
c. semicircular canals
d. ossicles
Solution:
B
Free Response
Exercise:
Problem:
How would a rise in altitude likely affect the speed of a sound
transmitted through air? Why?
Solution:
The sound would slow down, because it is transmitted through the
particles (gas) and there are fewer particles (lower density) at higher
altitudes.
Exercise:
Problem:
How might being in a place with less gravity than Earth has (such as
Earth’s moon) affect vestibular sensation, and why?
Solution:
Because vestibular sensation relies on gravity’s effects on tiny crystals
in the inner ear, a situation of reduced gravity would likely impair
vestibular sensation.
Glossary
audition
sense of hearing
basilar membrane
stiff structure in the cochlea that indirectly anchors auditory receptors
cochlea
whorled structure that contains receptors for transduction of the
mechanical wave into an electrical signal
incus
(also, anvil) second of the three bones of the middle ear
inner ear
innermost part of the ear; consists of the cochlea and the vestibular
system
labyrinth
bony, hollow structure that is the most internal part of the ear; contains
the sites of transduction of auditory and vestibular information
malleus
(also, hammer) first of the three bones of the middle ear
middle ear
part of the hearing apparatus that functions to transfer energy from the
tympanum to the oval window of the inner ear
organ of Corti
in the basilar membrane, the site of the transduction of sound, a
mechanical wave, to a neural signal
ossicle
one of the three bones of the middle ear
outer ear
part of the ear that consists of the pinna, ear canal, and tympanum and
which conducts sound waves into the middle ear
oval window
thin diaphragm between the middle and inner ears that receives sound
waves from contact with the stapes bone of the middle ear
pinna
cartilaginous outer ear
semicircular canal
one of three half-circular, fluid-filled tubes in the vestibular labyrinth
that monitors angular acceleration and deceleration
stapes
(also, stirrup) third of the three bones of the middle ear
stereocilia
in the auditory system, hair-like projections from hair cells that help
detect sound waves
tectorial membrane
cochlear structure that lies above the hair cells and participates in the
transduction of sound at the hair cells
tympanum
(also, tympanic membrane or ear drum) thin diaphragm between the
outer and middle ears
ultrasound
sound frequencies above the human detectable ceiling of
approximately 20,000 Hz
Vision
By the end of this section, you will be able to:
e Explain how electromagnetic waves differs from sound waves
e Trace the path of light through the eye to the point of the optic nerve
Vision is the ability to detect light patterns from the outside environment
and interpret them into images. Humans are bombarded with sensory
information, and the sheer volume of visual information can be
problematic. Fortunately, our visual system is able to attend to the most-
important stimuli. The importance of vision to humans is further
substantiated by the fact that about one-third of the human cerebral cortex is
dedicated to analyzing and perceiving visual information.
Light
As with auditory stimuli, light travels in waves. The compression waves
that compose sound must travel in a medium—a gas, a liquid, or a solid. In
contrast, light is composed of electromagnetic waves and needs no medium;
light can travel in a vacuum ((link]). The behavior of light can be discussed
in terms of the behavior of waves and also in terms of the behavior of the
fundamental unit of light—a packet of electromagnetic radiation called a
photon. A glance at the electromagnetic spectrum shows that visible light
for humans is just a small slice of the entire spectrum, which includes
radiation that we cannot see as light because it is below the frequency of
visible red light and above the frequency of visible violet light.
Certain variables are important when discussing perception of light.
Wavelength (which varies inversely with frequency) manifests itself as hue.
Light at the red end of the visible spectrum has longer wavelengths (and is
lower frequency), while light at the violet end has shorter wavelengths (and
is higher frequency). The wavelength of light is expressed in nanometers
(nm); one nanometer is one billionth of a meter. Humans perceive light that
ranges between approximately 380 nm and 740 nm. Some other animals,
though, can detect wavelengths outside of the human range. For example,
bees see near-ultraviolet light in order to locate nectar guides on flowers,
and some non-avian reptiles sense infrared light (heat that prey gives off).
Wavelength
(meters) Radio Microwave Infrared Visible Ultraviolet X-ray Gamma ray
10° 10? 10° 5x 10° 10° 107° 10
line a x ee @
Buildings Humans Honeybee Pinpoint Protozoans Molecules Atoms Atomic nuclei
I SSS
(Hz)
10* 108 10% 10% 10** 108 10”
In the electromagnetic spectrum, visible light lies between
380 nm and 740 nm. (credit: modification of work by
NASA)
Wave amplitude is perceived as luminous intensity, or brightness. The
standard unit of intensity of light is the candela, which is approximately the
luminous intensity of a one common candle.
Light waves travel 299,792 km per second in a vacuum, (and somewhat
slower in various media such as air and water), and those waves atrive at
the eye as long (red), medium (green), and short (blue) waves. What is
termed “white light” is light that is perceived as white by the human eye.
This effect is produced by light that stimulates equally the color receptors in
the human eye. The apparent color of an object is the color (or colors) that
the object reflects. Thus a red object reflects the red wavelengths in mixed
(white) light and absorbs all other wavelengths of light.
Anatomy of the Eye
The photoreceptive cells of the eye, where transduction of light to nervous
impulses occurs, are located in the retina (shown in [link]) on the inner
surface of the back of the eye. But light does not impinge on the retina
unaltered. It passes through other layers that process it so that it can be
interpreted by the retina ([link]b). The cornea, the front transparent layer of
the eye, and the crystalline lens, a transparent convex structure behind the
comea, both refract (bend) light to focus the image on the retina. The iris,
which is conspicuous as the colored part of the eye, is a circular muscular
ring lying between the lens and cornea that regulates the amount of light
entering the eye. In conditions of high ambient light, the iris contracts,
reducing the size of the pupil at its center. In conditions of low light, the iris
relaxes and the pupil enlarges.
Note:
Art Connection
Pupil
Iris Cornea Optic nerve
Aqueous humour
Ganglion
cells
Vitreous
humour
Amacrine
cells
Bipolar cells
Horizontal
cells
Optic nerve
Fovea
Retinal
blood vessels
(a) The human eye is shown in cross section. (b) A blowup
shows the layers of the retina.
The main function of the lens is to focus light on the retina and fovea
centralis. The lens is dynamic, focusing and re-focusing light as the eye
rests on near and far objects in the visual field. The lens is operated by
muscles that stretch it flat or allow it to thicken, changing the focal length
of light coming through it to focus it sharply on the retina. With age comes
the loss of the flexibility of the lens, and a form of farsightedness called
presbyopia results. Presbyopia occurs because the image focuses behind
the retina. Presbyopia is a deficit similar to a different type of
farsightedness called hyperopia caused by an eyeball that is too short. For
both defects, images in the distance are clear but images nearby are blurry.
Myopia (nearsightedness) occurs when an eyeball is elongated and the
image focus falls in front of the retina. In this case, images in the distance
are blurry but images nearby are clear.
There are two types of photoreceptors in the retina: rods and cones, named
for their general appearance as illustrated in [link]. Rods are strongly
photosensitive and are located in the outer edges of the retina. They detect
dim light and are used primarily for peripheral and nighttime vision. Cones
are weakly photosensitive and are located near the center of the retina. They
respond to bright light, and their primary role is in daytime, color vision.
Outer segment
contains rhodopsin
Rod outer
segment ee
Outer segment
contains
photopigments
Oil droplet
Nucleus
Rods and cones are photoreceptors
in the retina. Rods respond in low
light and can detect only shades of
gray. Cones respond in intense
light and are responsible for color
vision. (credit: modification of
work by Piotr Sliwa)
The fovea is the region in the center back of the eye that is responsible for
acute vision. The fovea has a high density of just cones. When you bring
your gaze to an object to examine it intently in bright light, the eyes orient
so that the object’s image falls on the fovea. This is the area of the retina
that gives us high clarity of vision. However, when looking at a star in the
night sky or other object in dim light, the object can be better viewed by the
peripheral vision because it is the rods in higher concentrations in the other
regions of the retina, rather than the cones at the center, that operate better
in low light. In low-light conditions, the rods allow us to see in shades of
gray because cones require bright light to be stimulated and don't respond in
low light conditions.
Note:
Link to Learning
ue
—
mess OPenstax COLLEGE
Review the anatomical structure of the eye, clicking on each part to
practice identification.
Processing Visual Input
Trichromatic Coding
There are three types of cones (with different photopsins), and they differ in
the wavelength to which they are most responsive, as shown in [link]. Some
cones are maximally responsive to short light waves of 420 nm, so they are
called S cones (“S” for “short”); others respond maximally to waves of 530
nm (M cones, for “medium”); a third group responds maximally to light of
longer wavelengths, at 560 nm (L, or “long” cones). With only one type of
cone, color vision would not be possible, and a two-cone (dichromatic)
system has limitations. Primates use a three-cone (trichromatic) system,
resulting in full color vision.
The color we perceive is a result of the ratio of activity of our three types of
cones. The colors of the visual spectrum, running from long-wavelength
light to short, are red (700 nm), orange (600 nm), yellow (565 nm), green
(497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans
have very sensitive perception of color and can distinguish about 500 levels
of brightness, 200 different hues, and 20 steps of saturation, or about 2
million distinct colors.
Normalized absorbance
400 500 600 700
Wavelength (nm)
Human rod cells and the different types of
cone cells each have an optimal
wavelength. However, there is
considerable overlap in the wavelengths
of light detected.
Retinal Processing
Visual signals leave the cones and rods, travel to the bipolar cells, and then
to ganglion cells. A large degree of processing of visual information occurs
in the retina itself, before visual information is sent to the brain.
Higher Processing
The myelinated axons of ganglion cells make up the optic nerves. Within
the nerves, different axons carry different qualities of the visual signal.
Some axons constitute the magnocellular (big cell) pathway, which carries
information about form, movement, depth, and differences in brightness.
Other axons constitute the parvocellular (small cell) pathway, which carries
information on color and fine detail. Some visual information projects
directly back into the brain, while other information crosses to the opposite
side of the brain. This crossing of optical pathways produces the distinctive
optic chiasma (Greek, for “crossing”) found at the base of the brain and
allows us to coordinate information from both eyes.
Once in the brain, visual information is processed in several places, and its
routes reflect the complexity and importance of visual information to
humans and other animals.
Section Summary
Vision is the only photo responsive sense. Visible light travels in waves and
is a very small slice of the electromagnetic radiation spectrum. Light waves
differ based on their frequency (wavelength = hue) and amplitude (intensity
= brightness).
In the vertebrate retina, there are two types of light receptors
(photoreceptors): cones and rods. Cones, which are the source of color
vision, exist in three forms—L, M, and S—and they are differentially
sensitive to different wavelengths. Cones are located in the retina, along
with the dim-light, achromatic receptors (rods). Cones are found in the
fovea, the central region of the retina, whereas rods are found everywhere
else throughout the retina.
Visual signals travel from the eye over the axons of retinal ganglion cells,
which make up the optic nerves. Ganglion cells come in several versions.
Some ganglion cell axons carry information on form, movement, depth, and
brightness, while other axons carry information on color and fine detail.
Art Connections
Exercise:
Problem:
[link] Which of the following statements about the human eye is false?
a. Rods detect color, while cones detect only shades of gray.
b. When light enters the retina, it passes the ganglion cells and
bipolar cells before reaching photoreceptors at the rear of the eye.
c. The iris adjusts the amount of light coming into the eye.
d. The cornea is a protective layer on the front of the eye.
Solution:
[link] A
Review Questions
Exercise:
Problem: Why do people over 55 often need reading glasses?
a. Their cornea no longer focuses correctly.
b. Their lens no longer focuses correctly.
c. Their eyeball has elongated with age, causing images to focus in
front of their retina.
d. Their retina has thinned with age, making vision more difficult.
Solution:
B
Exercise:
Problem:
Why is it easier to see images at night using peripheral, rather than the
central, vision?
a. Cones are denser in the periphery of the retina.
b. Bipolar cells are denser in the periphery of the retina.
c. Rods are denser in the periphery of the retina.
d. The optic nerve exits at the periphery of the retina.
Solution:
C
Glossary
candela
(cd) unit of measurement of luminous intensity (brightness)
circadian
describes a time cycle about one day in length
cone
weakly photosensitive, chromatic, cone-shaped neuron in the fovea of
the retina that detects bright light and is used in daytime color vision
cornea
transparent layer over the front of the eye that helps focus light waves
fovea
region in the center of the retina with a high density of photoreceptors
and which is responsible for acute vision
hyperopia
(also, farsightedness) visual defect in which the image focus falls
behind the retina, thereby making images in the distance clear, but
close-up images blurry
iris
pigmented, circular muscle at the front of the eye that regulates the
amount of light entering the eye
lens
transparent, convex structure behind the cornea that helps focus light
waves on the retina
myopia
(also, nearsightedness) visual defect in which the image focus falls in
front of the retina, thereby making images in the distance blurry, but
close-up images clear
presbyopia
visual defect in which the image focus falls behind the retina, thereby
making images in the distance clear, but close-up images blurry;
caused by age-based changes in the lens
pupil
small opening though which light enters
retina
layer of photoreceptive and supporting cells on the inner surface of the
back of the eye
rhodopsin
main photopigment in vertebrates
rod
strongly photosensitive, achromatic, cylindrical neuron in the outer
edges of the retina that detects dim light and is used in peripheral and
nighttime vision
superior colliculus
paired structure in the top of the midbrain, which manages eye
movements and auditory integration
suprachiasmatic nucleus
cluster of cells in the hypothalamus that plays a role in the circadian
cycle
tonic activity
in a neuron, slight continuous activity while at rest
vision
sense of sight
Introduction to the Immune System
class="introduction"
In this
compound
light
micrograph
purple-
Stained
neutrophil
(upper left)
and
eosinophil
(lower right)
are white
blood cells
that float
among red
blood cells
in this blood
smear.
Neutrophils
provide an
early, rapid,
and
nonspecific
defense
against
invading
pathogens.
Eosinophils
play a
variety of
roles in the
immune
response.
Red blood
cells are
about 7-8
pm in
diameter,
anda
neutrophil is
about 10—
12pm.
(credit:
modificatio
n of work
by Dr.
David
Csaba)
The environment consists of numerous pathogens, which are agents,
usually microorganisms, that cause diseases in their hosts. A host is the
organism that is invaded and often harmed by a pathogen. Pathogens
include bacteria, protists, fungi and other infectious organisms. We are
constantly exposed to pathogens in food and water, on surfaces, and in the
air. Mammalian immune systems evolved for protection from such
pathogens; they are composed of an extremely diverse array of specialized
cells and soluble molecules that coordinate a rapid and flexible defense
system capable of providing protection from a majority of these disease
agents.
Components of the immune system constantly search the body for signs of
pathogens. When pathogens are found, immune factors are mobilized to the
site of an infection. The immune factors identify the nature of the pathogen,
strengthen the corresponding cells and molecules to combat it efficiently,
and then halt the immune response after the infection is cleared to avoid
unnecessary host cell damage. The immune system can remember
pathogens to which it has been exposed to create a more efficient response
upon re-exposure. This memory can last several decades. Features of the
immune system, such as pathogen identification, specific response,
amplification, retreat, and remembrance are essential for survival against
pathogens. The immune response can be classified as either innate or active.
The innate immune response is always present and attempts to defend
against all pathogens rather than focusing on specific ones. Conversely, the
adaptive immune response stores information about past infections and
mounts pathogen-specific defenses.
Glossary
pathogen
an agent, usually a microorganism, that causes disease in the
organisms that they invade
host
an organism that is invaded by a pathogen or parasite
Innate Immunity
By the end of this section, you will be able to:
¢ Describe the body’s innate physical and chemical defenses
e Explain the inflammatory response
e Describe the complement system
The human immune system is a complex, multilayered system for
defending against external and internal threats to the integrity of the body.
The system can be divided into two types of defense systems: the innate
immune system, which is nonspecific toward a particular kind of pathogen,
and the adaptive immune system, which is specific ({link]). Innate
immunity is not caused by an infection or vaccination and depends initially
on physical and chemical barriers that work on all pathogens, sometimes
called the first line of defense. The second line of defense of the innate
system includes chemical signals that produce inflammation and fever
responses as well as mobilizing protective cells and other chemical
defenses. The adaptive immune system mounts a highly specific response to
substances and organisms that do not belong in the body. The adaptive
system takes longer to respond and has a memory system that allows it to
respond with greater intensity should the body reencounter a pathogen even
years later.
Vertebrate Immunity
Innate Immune System Adaptive Immune System
Physical Barriers Internal Defenses
¢ Skin, hair, cilia + Inflammatory response + Antibodies and the humoral immune response
* Mucus membranes * Complement proteins * Cell-mediated immune response
* Mucus and chemical secretions + Phagocytic cells * Memory response
* Digestive enzymes in mouth + Natural killer (NK) cells
* Stomach acid
There are two main parts to the vertebrate immune system. The
innate immune system, which is made up of physical barriers
and internal defenses, responds to all pathogens. The adaptive
immune system is highly specific.
External and Chemical Barriers
The body has significant physical barriers to potential pathogens. The skin
contains the protein keratin, which resists physical entry into cells. Other
body surfaces, particularly those associated with body openings, are
protected by the mucous membranes. The sticky mucus provides a physical
trap for pathogens, preventing their movement deeper into the body. The
openings of the body, such as the nose and ears, are protected by hairs that
catch pathogens, and the mucous membranes of the upper respiratory tract
have cilia that constantly move pathogens trapped in the mucus coat up to
the mouth.
The skin and mucous membranes also create a chemical environment that is
hostile to many microorganisms. The surface of the skin is acidic, which
prevents bacterial growth. Saliva, mucus, and the tears of the eye contain an
enzyme that breaks down bacterial cell walls. The stomach secretions create
a highly acidic environment, which kills many pathogens entering the
digestive system.
Finally, the surface of the body and the lower digestive system have a
community of microorganisms such as bacteria, archaea, and fungi that
coexist without harming the body. There is evidence that these organisms
are highly beneficial to their host, combating disease-causing organisms and
outcompeting them for nutritional resources provided by the host body.
Despite these defenses, pathogens may enter the body through skin
abrasions or punctures, or by collecting on mucosal surfaces in large
numbers that overcome the protections of mucus or cilia.
Internal Defenses
When pathogens enter the body, the innate immune system responds with a
variety of internal defenses. These include the inflammatory response,
phagocytosis, natural killer cells, and the complement system. White blood
cells in the blood and lymph recognize pathogens as foreign to the body. A
white blood cell is larger than a red blood cell, is nucleated, and is typically
able to move using amoeboid locomotion. Because they can move on their
own, white blood cells can leave the blood to go to infected tissues. For
example, a monocyte is a type of white blood cell that circulates in the
blood and lymph and develops into a macrophage after it moves into
infected tissue. A macrophage is a large cell that engulfs foreign particles
and pathogens. Mast cells are produced in the same way as white blood
cells, but unlike circulating white blood cells, mast cells take up residence
in connective tissues and especially mucosal tissues. They are responsible
for releasing chemicals in response to physical injury. They also play a role
in the allergic response, which will be discussed later in the chapter.
When a pathogen is recognized as foreign, chemicals called cytokines are
released. A cytokine is a chemical messenger that regulates cell
differentiation (form and function), proliferation (production), and gene
expression to produce a variety of immune responses. Approximately 40
types of cytokines exist in humans. In addition to being released from white
blood cells after pathogen recognition, cytokines are also released by the
infected cells and bind to nearby uninfected cells, inducing those cells to
release cytokines. This positive feedback loop results in a burst of cytokine
production.
One class of early-acting cytokines is the interferons, which are released by
infected cells as a warning to nearby uninfected cells. An interferon is a
small protein that signals a viral infection to other cells. The interferons
stimulate uninfected cells to produce compounds that interfere with viral
replication. Interferons also activate macrophages and other cells.
The Inflammatory Response and Phagocytosis
The first cytokines to be produced encourage inflammation, a localized
redness, swelling, heat, and pain. Inflammation is a response to physical
trauma, such as a cut or a blow, chemical irritation, and infection by
pathogens (viruses, bacteria, or fungi). The chemical signals that trigger an
inflammatory response enter the extracellular fluid and cause capillaries to
dilate (expand) and capillary walls to become more permeable, or leaky.
The serum and other compounds leaking from capillaries cause swelling of
the area, which in turn causes pain. Various kinds of white blood cells are
attracted to the area of inflammation. The types of white blood cells that
arrive at an inflamed site depend on the nature of the injury or infecting
pathogen. For example, a neutrophil is an early arriving white blood cell
that engulfs and digests pathogens. Neutrophils are the most abundant white
blood cells of the immune system ([Llink]). Macrophages follow neutrophils
and take over the phagocytosis function and are involved in the resolution
of an inflamed site, cleaning up cell debris and pathogens.
White blood cells
(leukocytes) release
chemicals to stimulate
the inflammatory
response following a
cut in the skin.
Cytokines also send feedback to cells of the nervous system to bring about
the overall symptoms of feeling sick, which include lethargy, muscle pain,
and nausea. Cytokines also increase the core body temperature, causing a
fever. The elevated temperatures of a fever inhibit the growth of pathogens
and speed up cellular repair processes. For these reasons, suppression of
fevers should be limited to those that are dangerously high.
Note:
Concept in Action
Opes aC)
4
Check out this 23-second, stop-motion video showing a neutrophil that
searches and engulfs fungus spores during an elapsed time of 79 minutes.
Natural Killer Cells
A lymphocyte is a white blood cell that contains a large nucleus ([link]).
Most lymphocytes are associated with the adaptive immune response, but
infected cells are identified and destroyed by natural killer cells, the only
lymphocytes of the innate immune system. A natural killer (NK) cell is a
lymphocyte that can kill cells infected with viruses (or cancerous cells). NK
cells identify intracellular infections, especially from viruses, by the altered
expression of major histocompatibility class (MHC) I molecules on the
surface of infected cells. MHC class I molecules are proteins on the
surfaces of all nucleated cells that provide a sample of the cell’s internal
environment at any given time. Unhealthy cells, whether infected or
cancerous, display an altered MHC class I complement on their cell
surfaces.
i,
Lymphocytes, such
as NK cells, are
characterized by
their large nuclei
that actively absorb
Wright stain and
therefore appear
dark colored under
a microscope.
(credit: scale-bar
data from Matt
Russell)
After the NK cell detects an infected or tumor cell, it induces programmed
cell death, or apoptosis. Phagocytic cells then come along and digest the
cell debris left behind. NK cells are constantly patrolling the body and are
an effective mechanism for controlling potential infections and preventing
cancer progression. The various types of immune cells are shown in [link].
Mast cell Natural killer cell Monocyte Macrophage Neutrophil
Cells involved in the innate immune response include mast
cells, natural killer cells, and white blood cells, such as
monocytes, macrophages and neutrophils.
Complement
An array of approximately 20 types of proteins, called a complement
system, is also activated by infection or the activity of the cells of the
adaptive immune system and functions to destroy extracellular pathogens.
Liver cells and macrophages synthesize inactive forms of complement
proteins continuously; these proteins are abundant in the blood serum and
are capable of responding immediately to infecting microorganisms. The
complement system is so named because it is complementary to the innate
and adaptive immune system. Complement proteins bind to the surfaces of
microorganisms and are particularly attracted to pathogens that are already
tagged by the adaptive immune system. This “tagging” involves the
attachment of specific proteins called antibodies (discussed in detail later)
to the pathogen. When they attach, the antibodies change shape providing a
binding site for one of the complement proteins. After the first few
complement proteins bind, a cascade of binding in a specific sequence of
proteins follows in which the pathogen rapidly becomes coated in
complement proteins.
Complement proteins perform several functions, one of which is to serve as
a marker to indicate the presence of a pathogen to phagocytic cells and
enhance engulfment. Certain complement proteins can combine to open
pores in microbial cell membranes and cause lysis of the cells.
Section Summary
The innate immune system consists first of physical and chemical barriers
to infection including the skin and mucous membranes and their secretions,
ciliated surfaces, and body hairs. The second line of defense is an internal
defense system designed to counter pathogenic threats that bypass the
physical and chemical barriers of the body. Using a combination of cellular
and molecular responses, the innate immune system identifies the nature of
a pathogen and responds with inflammation, phagocytosis, cytokine release,
destruction by NK cells, or the complement system.
Review Questions
Exercise:
Problem:
Which of the following is a barrier against pathogens provided by the
skin?
a. low pH
b. mucus
c. tears
d. cilia
Solution:
A
Exercise:
Problem:
Although interferons have several effects, they are particularly useful
against infections with which type of pathogen?
a. bacteria
b. viruses
c. fungi
d. helminths
Solution:
B
Exercise:
Problem:
Which innate immune system component uses MHC class I molecules
directly in its defense strategy?
a. macrophages
b. neutrophils
c. NK cells
d. interferon
Solution:
C
Free Response
Exercise:
Problem:
Different MHC class I molecules between donor and recipient cells
can lead to rejection of a transplanted organ or tissue. Suggest a reason
for this.
Solution:
If the MHC class I molecules expressed on donor cells differ from the
MHC class I molecules expressed on recipient cells, NK cells may
identify the donor cells as not normal and produce enzymes to induce
the donor cells to undergo apoptosis, which would destroy the
transplanted organ.
Exercise:
Problem:
If a series of genetic mutations prevented some, but not all, of the
complement proteins from binding antibodies or pathogens, would the
entire complement system be compromised?
Solution:
The entire complement system would probably be affected even when
only a few members were mutated such that they could no longer bind.
Because the complement involves the binding of activated proteins in
a specific sequence, when one or more proteins in the sequence is
absent, the subsequent proteins would be incapable of binding to elicit
the complement’s pathogen-destructive effects.
Glossary
complement system
an array of approximately 20 soluble proteins of the innate immune
system that enhance phagocytosis, bore holes in pathogens, and recruit
lymphocytes
cytokine
a chemical messenger that regulates cell differentiation, proliferation,
and gene expression to effect immune responses
inflammation
the localized redness, swelling, heat, and pain that results from the
movement of leukocytes through opened capillaries to a site of
infection
innate immunity
an immunity that occurs naturally because of genetic factors or
physiology, and is not caused by infection or vaccination
interferon
a cytokine that inhibits viral replication
lymphocyte
a type of white blood cell that includes natural killer cells of the innate
immune system and B and T cells of the adaptive immune system
macrophage
a large phagocytic cell that engulfs foreign particles and pathogens
major histocompatibility class (MHC) I
a group of proteins found on the surface of all nucleated cells that
signals to immune cells whether the cell is normal or is infected or
cancerous; it also provides the appropriate sites into which antigens
can be loaded for recognition by lymphocytes
mast cell
a leukocyte that produces inflammatory molecules, such as histamine,
in response to large pathogens
monocyte
a type of white blood cell that circulates in the blood and lymph and
differentiates into a macrophage after it moves into infected tissue
natural killer (NK) cell
a lymphocyte that can kill cells infected with viruses or tumor cells
neutrophil
a phagocytic leukocyte that engulfs and digests pathogens
white blood cell
a nucleated cell found in the blood that is a part of the immune system;
also called leukocytes
Adaptive Immunity
By the end of this section, you will be able to:
e Explain adaptive immunity
e Describe the cell-mediated immune response and humoral/antibody-
mediated immune response
The adaptive, or acquired, immune response takes days or even weeks to
become established—much longer than the innate response; however,
adaptive immunity is more specific to an invading pathogen. Adaptive
immunity is an immunity that occurs after exposure to an antigen either
from a pathogen or a vaccination. An antigen is a molecule that stimulates
a response in the immune system. This part of the immune system is
activated when the innate immune response is insufficient to control an
infection. In fact, without information from the innate immune system, the
adaptive response could not be mobilized. There are two types of adaptive
responses: the cell-mediated immune response, which is controlled by
activated T cells, and the humoral or antibody- immune response, which
is controlled by activated B cells and antibodies. Activated T and B cells
whose surface binding sites are specific to the molecules on the pathogen
greatly increase in numbers and attack the invading pathogen. Their attack
can kill pathogens directly or they can secrete antibodies that enhance the
phagocytosis of pathogens and disrupt the infection. Adaptive immunity
also involves a memory to give the host long-term protection from
reinfection with the same type of pathogen; on reexposure, this host
memory will facilitate a rapid and powerful response.
B and T Cells
Lymphocytes, which are white blood cells, are formed with other blood
cells in the red bone marrow found in many flat bones, such as the shoulder
or pelvic bones. The two types of lymphocytes of the adaptive immune
response are B and T cells ((link]). Whether an immature lymphocyte
becomes a B cell or T cell depends on where in the body it matures. The B
cells remain in the bone marrow to mature (hence the name “B” for “bone
marrow”), while T cells migrate to the thymus, where they mature (hence
the name “T” for “thymus”).
Maturation of a B or T cell involves becoming immunocompetent, meaning
that it can recognize, by binding, a specific molecule or antigen (discussed
below). During the maturation process, B and T cells that bind too strongly
to the body’s own cells are eliminated in order to minimize an immune
response against the body’s own tissues. Those cells that react weakly to the
body’s own cells, but have highly specific receptors on their cell surfaces
that allow them to recognize a foreign molecule, or antigen, remain. This
process occurs during fetal development and continues throughout life. The
specificity of this receptor is determined by the genetics of the individual
and is present before a foreign molecule is introduced to the body or
encountered. Thus, it is genetics and not experience that initially provides a
vast array of cells, each capable of binding to a different specific foreign
molecule. Once they are immunocompetent, the T and B cells will migrate
to the spleen and lymph nodes where they will remain until they are called
on during an infection. B cells are involved in the humoral immune
response, which targets pathogens loose in blood and lymph, and T cells are
involved in the cell-mediated immune response, which targets infected
cells.
This scanning electron
micrograph shows a T
lymphocyte. T and B
cells are indistinguishable
by light microscopy but
can be differentiated
experimentally by
probing their surface
receptors. (credit:
modification of work by
NCI; scale-bar data from
Matt Russell)
Humoral (or Antibody)-Mediated Immune Response
As mentioned, an antigen is a molecule that stimulates a response in the
immune system. Not every molecule is antigenic. B cells participate in a
chemical response to antigens present in the body by producing specific
antibodies that circulate throughout the body and bind with the antigen
whenever it is encountered. This is known as the humoral immune
response. As discussed, during maturation of B cells, a set of highly specific
B cells are produced that have many antigen receptor molecules in their
membrane ([link]).
Antigen
és Antigen-binding site
B cell receptors are
embedded in the
membranes of B
cells and bind a
variety of antigens
through their
variable regions.
Each B cell has only one kind of antigen receptor, which makes every B cell
different. Once the B cells mature in the bone marrow, they migrate to
lymph nodes or other lymphatic organs. When a B cell encounters the
antigen that binds to its receptor, the antigen molecule is brought into the
cell by endocytosis and reappears on the surface of the cell bound to an
MHC class II molecule. When this process is complete, the B cell is
sensitized. In most cases, the sensitized B cell must then encounter a
specific kind of T cell, called a helper T cell, before it is activated. The
helper T cell must already have been activated through an encounter with
the antigen (discussed below).
The helper T cell binds to the antigen-MHC class II complex and is induced
to release cytokines that induce the B cell to divide rapidly, which makes
thousands of identical (clonal) cells via mitosis. This process is called
clonal expansion. These daughter cells become either plasma cells or
memory B cells. The memory B cells remain inactive at this point, until
another later encounter with the antigen, caused by a reinfection by the
same bacteria or virus, results in them dividing into a new population of
plasma cells. The plasma cells, on the other hand, produce and secrete large
quantities, up to 100 million molecules per hour, of antibody molecules. An
antibody, also known as an immunoglobulin (Ig), is a protein that is
produced by plasma cells after stimulation by an antigen. Antibodies are the
agents of humoral immunity. Antibodies occur in the blood, in gastric and
mucus secretions, and in breast milk. Antibodies in these bodily fluids can
bind pathogens and mark them for destruction by phagocytes before they
can infect cells.
These antibodies circulate in the blood stream and lymphatic system and
bind with the antigen whenever it is encountered. The binding can fight
infection in several ways. Antibodies can bind to viruses or bacteria and
interfere with the chemical interactions required for them to infect or bind
to other cells. The antibodies may create bridges between different particles
containing antigenic sites clumping them all together and preventing their
proper functioning. The antigen-antibody complex stimulates the
complement system described previously, destroying the cell bearing the
antigen. Phagocytic cells, such as those already described, are attracted by
the antigen-antibody complexes, and phagocytosis is enhanced when the
complexes are present. Finally, antibodies stimulate inflammation, and their
presence in mucus and on the skin prevents pathogen attack.
Antibodies coat extracellular pathogens and neutralize them by blocking
key sites on the pathogen that enhance their infectivity (such as receptors
that “dock” pathogens on host cells) ({link]). Antibody neutralization can
prevent pathogens from entering and infecting host cells. The neutralized
antibody-coated pathogens can then be filtered by the spleen and eliminated
in urine or feces.
Antibodies also mark pathogens for destruction by phagocytic cells, such as
macrophages or neutrophils, in a process called opsonization. In a process
called complement fixation, some antibodies provide a place for
complement proteins to bind. The combination of antibodies and
complement promotes rapid clearing of pathogens.
The production of antibodies by plasma cells in response to an antigen is
called active immunity and describes the host’s active response of the
immune system to an infection or to a vaccination. There is also a passive
immune response where antibodies come from an outside source, instead of
the individual’s own plasma cells, and are introduced into the host. For
example, antibodies circulating in a pregnant woman’s body move across
the placenta into the developing fetus. The child benefits from the presence
of these antibodies for up to several months after birth. In addition, a
passive immune response is possible by injecting antibodies into an
individual in the form of an antivenom to a snake-bite toxin or antibodies in
blood serum to help fight a hepatitis infection. This gives immediate
protection since the body does not need the time required to mount its own
response.
(a) Neutralization Antibodies prevent a virus or toxic protein
from binding their target.
Antibody
Virus
Diptheria toxin
(b) Opsonization A pathogen tagged by antibodies is consumed
by a macrophage or neutrophil.
Macrophage
Pathogen
(c) Complement activation Antibodies attached to the surface
of a pathogen cell activate the complement system.
Pores formed
by complement
Antibodies may inhibit infection
by (a) preventing the antigen
from binding its target, (b)
tagging a pathogen for
destruction by macrophages or
neutrophils, or (c) activating the
complement cascade.
Cell-Mediated Immunity
Unlike B cells, T lymphocytes are unable to recognize pathogens without
assistance. Instead, dendritic cells and macrophages first engulf and digest
pathogens into hundreds or thousands of antigens. Then, an antigen-
presenting cell (APC) detects, engulfs, and informs the adaptive immune
response about an infection. When a pathogen is detected, these APCs will
engulf and break it down through phagocytosis. Antigen fragments will
then be transported to the surface of the APC, where they will serve as an
indicator to other immune cells. A dendritic cell is an immune cell that
mops up antigenic materials in its surroundings and presents them on its
surface. Dendritic cells are located in the skin, the linings of the nose, lungs,
stomach, and intestines. These positions are ideal locations to encounter
invading pathogens. Once they are activated by pathogens and mature to
become APCs they migrate to the spleen or a lymph node. Macrophages
also function as APCs. After phagocytosis by a macrophage, the phagocytic
vesicle fuses with an intracellular lysosome. Within the resulting
phagolysosome, the components are broken down into fragments; the
fragments are then loaded onto MHC class II molecules and are transported
to the cell surface for antigen presentation ([{link]). Helper T cells cannot
properly respond to an antigen unless it is processed and embedded in an
MHC class II molecule. The APCs express MHC class II on their surfaces,
and when combined with a foreign antigen, these complexes signal an
invader.
(2) Abacterium
engulfed by a
macrophage is
encased ina
vacuole. (2) Lysosomes fuse
with the vacuole
and digest the
Antigen bacterium.
Macrophage
(3) Antigens from digested
bacterium are presented with
MHC II on the cell surface.
An antigen-presenting cell
(APC), such as a macrophage,
engulfs a foreign antigen,
partially digests it in a lysosome,
and then embeds it in an MHC
class II molecule for
presentation at the cell surface.
Lymphocytes of the adaptive
immune response must interact
with antigen-embedded MHC
class IT molecules to mature into
functional immune cells.
Note:
Concept in Action
View this animation from Rockefeller University to see how dendritic cells
act as sentinels in the body’s immune system.
T cells have many functions. Some respond to APCs of the innate immune
system and indirectly induce immune responses by releasing cytokines.
Others stimulate B cells to start the humoral response as described
previously. Another type of T cell detects APC signals and directly kills the
infected cells, while some are involved in suppressing inappropriate
immune reactions to harmless or “self” antigens.
There are two main types of T cells: helper T lymphocytes (T}) and the
cytotoxic T lymphocytes (Tc). The Ty lymphocytes function indirectly to
tell other immune cells about potential pathogens.
Cytotoxic T cells (Tc) are the key component of the cell-mediated part of
the adaptive immune system and attack and destroy infected cells. Tc cells
are particularly important in protecting against viral infections; this is
because viruses replicate within cells where they are shielded from
extracellular contact with circulating antibodies. Once activated, the Tc
creates a large clone of cells with one specific set of cell-surface receptors,
as in the case with clonal expansion of activated B cells. As with B cells,
the clone includes active T¢ cells and inactive memory T¢ cells. The
resulting active Tc cells then identify infected host cells. Because of the
time required to generate a population of clonal T and B cells, there is a
delay in the adaptive immune response compared to the innate immune
response.
Tc cells attempt to identify and destroy infected cells before the pathogen
can replicate and escape, thereby halting the progression of intracellular
infections. Tc cells also support NK lymphocytes to destroy early cancers.
Cytokines secreted by the T};1 response that stimulates macrophages also
stimulate Tc cells and enhance their ability to identify and destroy infected
cells and tumors. A summary of how the humoral and cell-mediated
immune responses are activated appears in [link].
B plasma cells and T¢ cells are collectively called effector cells because
they are involved in “effecting” (bringing about) the immune response of
killing pathogens and infected host cells.
Bacterium
Antigen >
a
Macrophage
Antigens from digested
bacterium are presented with
MHC II on the cell surface. In response to cytokines,
the T cell clones itself.
Activated
helper T cell
Humoral
; —, immune
>
clo
L X .. j response
ee — cell clones itself
Cell-mediated
immune
response
“2 Cytokines
; "2 T cell becomes
Cytokines activate cytotoxic.
B cells and T cells
A helper T cell becomes activated by binding to an antigen
presented by an APC via the MHCII receptor, causing it to
release cytokines. Depending on the cytokines released, this
activates either the humoral or the cell-mediated immune
response.
Immunological Memory
The adaptive immune system has a memory component that allows for a
rapid and large response upon reinvasion of the same pathogen. During the
adaptive immune response to a pathogen that has not been encountered
before, known as the primary immune response, plasma cells secreting
antibodies and differentiated T cells increase, then plateau over time. As B
and T cells mature into effector cells, a subset of the naive populations
differentiates into B and T memory cells with the same antigen specificities
({link]). A memory cell is an antigen-specific B or T lymphocyte that does
not differentiate into an effector cell during the primary immune response,
but that can immediately become an effector cell on reexposure to the same
pathogen. As the infection is cleared and pathogenic stimuli subside, the
effectors are no longer needed and they undergo apoptosis. In contrast, the
memory cells persist in the circulation.
Note:
Art Connection
B cell receptor
Antigen on
bacterium
B cell
Helper T cell
AGE Cytokines
T cell receptor
(oy
~) Ay Tt
: gps»
Memory B cells Plasma cells
After initially binding an antigen to
the B cell receptor, a B cell
internalizes the antigen and
presents it on MHC class II. A
helper T cell recognizes the MHC
class II- antigen complex and
activates the B cell. As a result,
memory B cells and plasma cells
are made.
If the pathogen is never encountered again during the individual’s lifetime,
B and T memory cells will circulate for a few years or even several decades
and will gradually die off, having never functioned as effector cells.
However, if the host is re-exposed to the same pathogen type, circulating
memory cells will immediately differentiate into plasma cells and T¢ cells
without input from APCs or Ty cells. This is known as the secondary
immune response. One reason why the adaptive immune response is
delayed is because it takes time for naive B and T cells with the appropriate
antigen specificities to be identified, activated, and proliferate. On
reinfection, this step is skipped, and the result is a more rapid production of
immune defenses. Memory B cells that differentiate into plasma cells
output tens to hundreds-fold greater antibody amounts than were secreted
during the primary response ((link]). This rapid and dramatic antibody
response may stop the infection before it can even become established, and
the individual may not realize they had been exposed.
Antibody concentration ——»
Primary immune Secondary immune
response response
Time ——>
In the primary response to
infection, antibodies are
secreted first from plasma
cells. Upon re-exposure to
the same pathogen, memory
cells differentiate into
antibody-secreting plasma
cells that output a greater
amount of antibody for a
longer period of time.
Vaccination is based on the knowledge that exposure to noninfectious
antigens, derived from known pathogens, generates a mild primary immune
response. The immune response to vaccination may not be perceived by the
host as illness but still confers immune memory. When exposed to the
corresponding pathogen to which an individual was vaccinated, the reaction
is similar to a secondary exposure. Because each reinfection generates more
memory cells and increased resistance to the pathogen, some vaccine
courses involve one or more booster vaccinations to mimic repeat
exposures.
The Lymphatic System
Lymph is the watery fluid that bathes tissues and organs and contains
protective white blood cells but does not contain erythrocytes. Lymph
moves about the body through the lymphatic system, which is made up of
vessels, lymph ducts, lymph glands, and organs, such as tonsils, adenoids,
thymus, and spleen.
Although the immune system is characterized by circulating cells
throughout the body, the regulation, maturation, and intercommunication of
immune factors occur at specific sites. The blood circulates immune cells,
proteins, and other factors through the body. Approximately 0.1 percent of
all cells in the blood are leukocytes, which include monocytes (the
precursor of macrophages) and lymphocytes. Most cells in the blood are red
blood cells. Cells of the immune system can travel between the distinct
lymphatic and blood circulatory systems, which are separated by interstitial
space, by a process called extravasation (passing through to surrounding
tissue).
Recall that cells of the immune system originate from stem cells in the bone
marrow. B cell maturation occurs in the bone marrow, whereas progenitor
cells migrate from the bone marrow and develop and mature into naive T
cells in the organ called the thymus.
On maturation, T and B lymphocytes circulate to various destinations.
Lymph nodes scattered throughout the body house large populations of T
and B cells, dendritic cells, and macrophages ([link]). Lymph gathers
antigens as it drains from tissues. These antigens then are filtered through
lymph nodes before the lymph is returned to circulation. APCs in the lymph
nodes capture and process antigens and inform nearby lymphocytes about
potential pathogens.
Afferent lymphatic
vessel
Efferent lymphatic
vessel
(b)
(a) Lymphatic vessels carry a clear fluid called
lymph throughout the body. The liquid passes
through (b) lymph nodes that filter the lymph
that enters the node through afferent vessels
and leaves through efferent vessels; lymph
nodes are filled with lymphocytes that purge
infecting cells. (credit a: modification of work
by NIH; credit b: modification of work by NCI,
NIH)
The spleen houses B and T cells, macrophages, dendritic cells, and NK cells
({link]). The spleen is the site where APCs that have trapped foreign
particles in the blood can communicate with lymphocytes. Antibodies are
synthesized and secreted by activated plasma cells in the spleen, and the
spleen filters foreign substances and antibody-complexed pathogens from
the blood. Functionally, the spleen is to the blood as lymph nodes are to the
lymph.
The spleen functions to
immunologically filter the
blood and allow for
communication between cells
corresponding to the innate
and adaptive immune
responses. (credit:
modification of work by NCI,
NIH)
Section Summary
The adaptive immune response is a slower-acting, longer-lasting, and more
specific response than the innate response. However, the adaptive response
requires information from the innate immune system to function. APCs
display antigens on MHC molecules to naive T cells. T cells with cell-
surface receptors that bind a specific antigen will bind to that APC. In
response, the T cells differentiate and proliferate, becoming Ty cells or Tc
cells. Ty cells stimulate B cells that have engulfed and presented pathogen-
derived antigens. B cells differentiate into plasma cells that secrete
antibodies, whereas T¢ cells destroy infected or cancerous cells. Memory
cells are produced by activated and proliferating B and T cells and persist
after a primary exposure to a pathogen. If re-exposure occurs, memory cells
differentiate into effector cells without input from the innate immune
system.
Art Connections
Exercise:
Problem:
[link] The Rh antigen is found on Rh-positive red blood cells. An Rh-
negative female can usually carry an Rh-positive fetus to term without
difficulty. However, if she has a second Rh-positive fetus, her body
may launch an immune attack that causes hemolytic disease of the
newborn. Why do you think hemolytic disease is only a problem
during the second or subsequent pregnancies?
Solution:
[link] If the blood of the mother and fetus mixes, memory cells that
recognize the Rh antigen of the fetus can form in the mother late in the
first pregnancy. During subsequent pregnancies, these memory cells
launch an immune attack on the fetal blood cells of an Rh-positive
fetus. Injection of anti-Rh antibody during the first pregnancy prevents
the immune response from occurring.
Review Questions
Exercise:
Problem:The humoral immune response depends on which cells?
a. Tc cells
b. B cells
c. B and Ty cells
d. Tc and Ty cells
Solution:
C
Exercise:
Problem:
Foreign particles circulating in the blood are filtered by the
a. spleen
b. lymph nodes
c. MALT
d. lymph
Solution:
A
Free Response
Exercise:
Problem:
How do B and T cells differ with respect to antigens that they bind?
Solution:
T cells bind antigens that have been digested and embedded in MHC
molecules by APCs. In contrast, B cells function as APCs to bind
intact, unprocessed antigens.
Exercise:
Problem:
Why is the immune response after reinfection much faster than the
adaptive immune response after the initial infection?
Solution:
Upon reinfection, the memory cells will immediately differentiate into
plasma cells and CTLs without input from APCs or Ty cells. In
contrast, the adaptive immune response to the initial infection requires
time for naive B and T cells with the appropriate antigen specificities
to be identified and activated.
Glossary
active immunity
an immunity that occurs as a result of the activity of the body’s own
cells rather than from antibodies acquired from an external source
adaptive immunity
a specific immune response that occurs after exposure to an antigen
either from a pathogen or a vaccination
antibody
a protein that is produced by plasma cells after stimulation by an
antigen; also known as an immunoglobulin
antigen
a macromolecule that reacts with cells of the immune system and
which may or may not have a stimulatory effect
antigen-presenting cell (APC)
an immune cell that detects, engulfs, and informs the adaptive immune
response about an infection by presenting the processed antigen on its
cell surface
B cell
a lymphocyte that matures in the bone marrow
cell-mediated immune response
an adaptive immune response that is controlled by T cells
cytotoxic T lymphocyte (Tc)
an adaptive immune cell that directly kills infected cells via enzymes,
and that releases cytokines to enhance the immune response
dendritic cell
an immune cell that processes antigen material and presents it on the
surface of its cell in MHC class II molecules and induces an immune
response in other cells
effector cell
a lymphocyte that has differentiated, such as a B cell, plasma cell, or
cytotoxic T cell
helper T lymphocyte (Ty)
a cell of the adaptive immune system that binds APCs via MHC class
II molecules and stimulates B cells or secretes cytokines to initiate the
immune response
humoral immune response
the adaptive immune response that is controlled by activated B cells
and antibodies
immune tolerance
an acquired ability to prevent an unnecessary or harmful immune
response to a detected foreign body known not to cause disease
lymph
the watery fluid present in the lymphatic circulatory system that bathes
tissues and organs with protective white blood cells and does not
contain erythrocytes
memory cell
an antigen-specific B or T lymphocyte that does not differentiate into
an effector cell during the primary immune response but that can
immediately become an effector cell on reexposure to the same
pathogen
major histocompatibility class (MHC) II molecule
a protein found on the surface of antigen-presenting cells that signals
to immune cells whether the cell is normal or is infected or cancerous;
it provides the appropriate template into which antigens can be loaded
for recognition by lymphocytes
passive immunity
an immunity that does not result from the activity of the body’s own
immune cells but by transfer of antibodies from one individual to
another
primary immune response
the response of the adaptive immune system to the first exposure to an
antigen
secondary immune response
the response of the adaptive immune system to a second or later
exposure to an antigen mediated by memory cells
T cell
a lymphocyte that matures in the thymus gland
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